| Message ID | 20190812173019.11956-21-julien.grall@arm.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | xen/arm: Rework head.S to make it more compliant with the Arm Arm | expand |
On Mon, 12 Aug 2019, Julien Grall wrote: > The 1:1 mapping may clash with other parts of the Xen virtual memory > layout. At the moment, Xen is handling the clash by only creating a > mapping to the runtime virtual address before enabling the MMU. > > The rest of the mappings (such as the fixmap) will be mapped after the > MMU is enabled. However, the code doing the mapping is not safe as it > replace mapping without using the Break-Before-Make sequence. > > As the 1:1 mapping can be anywhere in the memory, it is easier to remove > all the entries added as soon as the 1:1 mapping is not used rather than > adding the Break-Before-Make sequence everywhere. > > It is difficult to track where exactly the 1:1 mapping was created > without a full rework of create_page_tables(). Instead, introduce a new > function remove_identity_mapping() will look where is the top-level entry > for the 1:1 mapping and remove it. > > The new function is only called for the boot CPU. Secondary CPUs will > switch directly to the runtime page-tables so there are no need to > remove the 1:1 mapping. Note that this still doesn't make the Secondary > CPUs path safe but it is not making it worst. > > Signed-off-by: Julien Grall <julien.grall@arm.com> > > --- > It is very likely we will need to re-introduce the 1:1 mapping to cater > secondary CPUs boot and suspend/resume. For now, the attempt is to make > boot CPU path fully Arm Arm compliant. > > Changes in v3: > - Remove unused label > - Avoid harcoding slots > > Changes in v2: > - Patch added > --- > xen/arch/arm/arm32/head.S | 86 +++++++++++++++++++++++++++++++++++++---------- > 1 file changed, 69 insertions(+), 17 deletions(-) > > diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S > index 8f945d318a..34fc9ffcee 100644 > --- a/xen/arch/arm/arm32/head.S > +++ b/xen/arch/arm/arm32/head.S > @@ -32,6 +32,10 @@ > #define PT_UPPER(x) (PT_##x & 0xf00) > #define PT_LOWER(x) (PT_##x & 0x0ff) > > +/* Convenience defines to get slot used by Xen mapping. */ > +#define XEN_FIRST_SLOT first_table_offset(XEN_VIRT_START) > +#define XEN_SECOND_SLOT second_table_offset(XEN_VIRT_START) > + > #if (defined (CONFIG_EARLY_PRINTK)) && (defined (EARLY_PRINTK_INC)) > #include EARLY_PRINTK_INC > #endif > @@ -158,6 +162,13 @@ past_zImage: > ldr r0, =primary_switched > mov pc, r0 > primary_switched: > + /* > + * The 1:1 map may clash with other parts of the Xen virtual memory > + * layout. As it is not used anymore, remove it completely to > + * avoid having to worry about replacing existing mapping > + * afterwards. > + */ > + bl remove_identity_mapping > bl setup_fixmap > #ifdef CONFIG_EARLY_PRINTK > /* Use a virtual address to access the UART. */ > @@ -474,12 +485,63 @@ enable_mmu: > mov pc, lr > ENDPROC(enable_mmu) > > -setup_fixmap: > +/* > + * Remove the 1:1 map for the page-tables. It is not easy to keep track > + * where the 1:1 map was mapped, so we will look for the top-level entry > + * exclusive to the 1:1 map and remove it. > + * > + * Inputs: > + * r9 : paddr(start) > + * > + * Clobbers r0 - r3 > + */ > +remove_identity_mapping: > + /* r2:r3 := invalid page-table entry */ > + mov r2, #0x0 > + mov r3, #0x0 > /* > - * Now we can install the fixmap and dtb mappings, since we > - * don't need the 1:1 map any more > + * Find the first slot used. Remove the entry for the first > + * table if the slot is not XEN_FIRST_SLOT. For slot XEN_FIRST_SLOT, > + * the 1:1 mapping was done in the second table. May I suggest something more similar to the arm64 version as a comment? Something like: "Find the first slot used. Remove the entry for the first table if the slot is not XEN_FIRST_SLOT. In case of XEN_FIRST_SLOT, we have to look at the second table as the slot is shared with the XEN_VIRT_START mapping." > */ > - dsb > + lsr r1, r9, #FIRST_SHIFT > + mov_w r0, LPAE_ENTRY_MASK ldr? > + and r1, r1, r0 /* r1 := first slot */ > + cmp r1, #XEN_FIRST_SLOT NIT: align > + beq 1f > + /* It is not in slot 0, remove the entry */ > + ldr r0, =boot_pgtable /* r0 := root table */ > + lsl r1, r1, #3 /* r1 := Slot offset */ > + strd r2, r3, [r0, r1] > + b identity_mapping_removed > + > +1: > + /* > + * Find the second slot used. Remove the entry for the first > + * table if the slot is not XEN_SECOND_SLOT. For slot XEN_SECOND_SLOT, > + * it means the 1:1 mapping was not created. > + */ > + lsr r1, r9, #SECOND_SHIFT > + mov_w r0, LPAE_ENTRY_MASK > + and r1, r1, r0 /* r1 := second slot */ > + cmp r1, #XEN_SECOND_SLOT > + beq identity_mapping_removed > + /* It is not in slot 1, remove the entry */ > + ldr r0, =boot_second /* r0 := second table */ > + lsl r1, r1, #3 /* r1 := Slot offset */ > + strd r2, r3, [r0, r1] > + > +identity_mapping_removed: > + /* See asm-arm/arm32/flushtlb.h for the explanation of the sequence. */ > + dsb nshst > + mcr CP32(r0, TLBIALLH) > + dsb nsh > + isb As for the arm64 patch, I would like to understand dsb ishst vs. dsb nshst. > + mov pc, lr > +ENDPROC(remove_identity_mapping) > + > +setup_fixmap: > #if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */ > /* Add UART to the fixmap table */ > ldr r1, =xen_fixmap /* r1 := vaddr (xen_fixmap) */ > @@ -489,7 +551,6 @@ setup_fixmap: > orr r2, r2, #PT_LOWER(DEV_L3) /* r2:r3 := 4K dev map including UART */ > mov r3, #0x0 > strd r2, r3, [r1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's slot */ > -1: > > /* Map fixmap into boot_second */ > ldr r1, =boot_second /* r1 := vaddr (boot_second) */ > @@ -501,19 +562,10 @@ setup_fixmap: > mov r4, r4, lsr #(SECOND_SHIFT - 3) /* r4 := Slot for FIXMAP(0) */ > mov r3, #0x0 > strd r2, r3, [r1, r4] /* Map it in the fixmap's slot */ > -#endif > - > - /* > - * Flush the TLB in case the 1:1 mapping happens to clash with > - * the virtual addresses used by the fixmap or DTB. > - */ > - dsb /* Ensure any page table updates made above > - * have occurred. */ > > - isb > - mcr CP32(r0, TLBIALLH) /* Flush hypervisor TLB */ > - dsb /* Ensure completion of TLB flush */ > - isb > + /* Ensure any page table updates made above have occurred. */ > + dsb nshst same here > +#endif > mov pc, lr > ENDPROC(setup_fixmap) > > -- > 2.11.0 >
Hi Stefano, On 8/22/19 7:17 PM, Stefano Stabellini wrote: > On Mon, 12 Aug 2019, Julien Grall wrote: >> The 1:1 mapping may clash with other parts of the Xen virtual memory >> layout. At the moment, Xen is handling the clash by only creating a >> mapping to the runtime virtual address before enabling the MMU. >> >> The rest of the mappings (such as the fixmap) will be mapped after the >> MMU is enabled. However, the code doing the mapping is not safe as it >> replace mapping without using the Break-Before-Make sequence. >> >> As the 1:1 mapping can be anywhere in the memory, it is easier to remove >> all the entries added as soon as the 1:1 mapping is not used rather than >> adding the Break-Before-Make sequence everywhere. >> >> It is difficult to track where exactly the 1:1 mapping was created >> without a full rework of create_page_tables(). Instead, introduce a new >> function remove_identity_mapping() will look where is the top-level entry >> for the 1:1 mapping and remove it. >> >> The new function is only called for the boot CPU. Secondary CPUs will >> switch directly to the runtime page-tables so there are no need to >> remove the 1:1 mapping. Note that this still doesn't make the Secondary >> CPUs path safe but it is not making it worst. >> >> Signed-off-by: Julien Grall <julien.grall@arm.com> >> >> --- >> It is very likely we will need to re-introduce the 1:1 mapping to cater >> secondary CPUs boot and suspend/resume. For now, the attempt is to make >> boot CPU path fully Arm Arm compliant. >> >> Changes in v3: >> - Remove unused label >> - Avoid harcoding slots >> >> Changes in v2: >> - Patch added >> --- >> xen/arch/arm/arm32/head.S | 86 +++++++++++++++++++++++++++++++++++++---------- >> 1 file changed, 69 insertions(+), 17 deletions(-) >> >> diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S >> index 8f945d318a..34fc9ffcee 100644 >> --- a/xen/arch/arm/arm32/head.S >> +++ b/xen/arch/arm/arm32/head.S >> @@ -32,6 +32,10 @@ >> #define PT_UPPER(x) (PT_##x & 0xf00) >> #define PT_LOWER(x) (PT_##x & 0x0ff) >> >> +/* Convenience defines to get slot used by Xen mapping. */ >> +#define XEN_FIRST_SLOT first_table_offset(XEN_VIRT_START) >> +#define XEN_SECOND_SLOT second_table_offset(XEN_VIRT_START) >> + >> #if (defined (CONFIG_EARLY_PRINTK)) && (defined (EARLY_PRINTK_INC)) >> #include EARLY_PRINTK_INC >> #endif >> @@ -158,6 +162,13 @@ past_zImage: >> ldr r0, =primary_switched >> mov pc, r0 >> primary_switched: >> + /* >> + * The 1:1 map may clash with other parts of the Xen virtual memory >> + * layout. As it is not used anymore, remove it completely to >> + * avoid having to worry about replacing existing mapping >> + * afterwards. >> + */ >> + bl remove_identity_mapping >> bl setup_fixmap >> #ifdef CONFIG_EARLY_PRINTK >> /* Use a virtual address to access the UART. */ >> @@ -474,12 +485,63 @@ enable_mmu: >> mov pc, lr >> ENDPROC(enable_mmu) >> >> -setup_fixmap: >> +/* >> + * Remove the 1:1 map for the page-tables. It is not easy to keep track >> + * where the 1:1 map was mapped, so we will look for the top-level entry >> + * exclusive to the 1:1 map and remove it. >> + * >> + * Inputs: >> + * r9 : paddr(start) >> + * >> + * Clobbers r0 - r3 >> + */ >> +remove_identity_mapping: >> + /* r2:r3 := invalid page-table entry */ >> + mov r2, #0x0 >> + mov r3, #0x0 >> /* >> - * Now we can install the fixmap and dtb mappings, since we >> - * don't need the 1:1 map any more >> + * Find the first slot used. Remove the entry for the first >> + * table if the slot is not XEN_FIRST_SLOT. For slot XEN_FIRST_SLOT, >> + * the 1:1 mapping was done in the second table. > > May I suggest something more similar to the arm64 version as a comment? > Something like: > > "Find the first slot used. Remove the entry for the first table if the > slot is not XEN_FIRST_SLOT. In case of XEN_FIRST_SLOT, we have to look > at the second table as the slot is shared with the XEN_VIRT_START > mapping." I have answered that on the arm64 version. Let's continue the conversation there. > > >> */ >> - dsb >> + lsr r1, r9, #FIRST_SHIFT >> + mov_w r0, LPAE_ENTRY_MASK > > ldr? What's wrong with the mov_w? Ok it is two instructions but... the constant will be stored in a literal and therefore induce a memory load (see patch #8). > > >> + and r1, r1, r0 /* r1 := first slot */ >> + cmp r1, #XEN_FIRST_SLOT > > NIT: align align of? > > >> + beq 1f >> + /* It is not in slot 0, remove the entry */ >> + ldr r0, =boot_pgtable /* r0 := root table */ >> + lsl r1, r1, #3 /* r1 := Slot offset */ >> + strd r2, r3, [r0, r1] >> + b identity_mapping_removed >> + >> +1: >> + /* >> + * Find the second slot used. Remove the entry for the first >> + * table if the slot is not XEN_SECOND_SLOT. For slot XEN_SECOND_SLOT, >> + * it means the 1:1 mapping was not created. >> + */ >> + lsr r1, r9, #SECOND_SHIFT >> + mov_w r0, LPAE_ENTRY_MASK >> + and r1, r1, r0 /* r1 := second slot */ >> + cmp r1, #XEN_SECOND_SLOT >> + beq identity_mapping_removed >> + /* It is not in slot 1, remove the entry */ >> + ldr r0, =boot_second /* r0 := second table */ >> + lsl r1, r1, #3 /* r1 := Slot offset */ >> + strd r2, r3, [r0, r1] >> + >> +identity_mapping_removed: >> + /* See asm-arm/arm32/flushtlb.h for the explanation of the sequence. */ >> + dsb nshst >> + mcr CP32(r0, TLBIALLH) >> + dsb nsh >> + isb > > As for the arm64 patch, I would like to understand dsb ishst vs. dsb > nshst. I have answered that on the arm64 version. Let's continue the conversation there. > > >> + mov pc, lr >> +ENDPROC(remove_identity_mapping) >> + >> +setup_fixmap: >> #if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */ >> /* Add UART to the fixmap table */ >> ldr r1, =xen_fixmap /* r1 := vaddr (xen_fixmap) */ >> @@ -489,7 +551,6 @@ setup_fixmap: >> orr r2, r2, #PT_LOWER(DEV_L3) /* r2:r3 := 4K dev map including UART */ >> mov r3, #0x0 >> strd r2, r3, [r1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's slot */ >> -1: >> >> /* Map fixmap into boot_second */ >> ldr r1, =boot_second /* r1 := vaddr (boot_second) */ >> @@ -501,19 +562,10 @@ setup_fixmap: >> mov r4, r4, lsr #(SECOND_SHIFT - 3) /* r4 := Slot for FIXMAP(0) */ >> mov r3, #0x0 >> strd r2, r3, [r1, r4] /* Map it in the fixmap's slot */ >> -#endif >> - >> - /* >> - * Flush the TLB in case the 1:1 mapping happens to clash with >> - * the virtual addresses used by the fixmap or DTB. >> - */ >> - dsb /* Ensure any page table updates made above >> - * have occurred. */ >> >> - isb >> - mcr CP32(r0, TLBIALLH) /* Flush hypervisor TLB */ >> - dsb /* Ensure completion of TLB flush */ >> - isb >> + /* Ensure any page table updates made above have occurred. */ >> + dsb nshst > > same here I have answered that on the arm64 version. Let's continue the conversation there. > > >> +#endif >> mov pc, lr >> ENDPROC(setup_fixmap) >> >> -- >> 2.11.0 >> Cheers,
On Thu, 22 Aug 2019, Julien Grall wrote: > > > */ > > > - dsb > > > + lsr r1, r9, #FIRST_SHIFT > > > + mov_w r0, LPAE_ENTRY_MASK > > > > ldr? > > What's wrong with the mov_w? Ok it is two instructions but... the constant > will be stored in a literal and therefore induce a memory load (see patch #8). I am just wondering why you would choose mov_w when you can just do a single simple ldr. > > > > > + and r1, r1, r0 /* r1 := first slot */ > > > + cmp r1, #XEN_FIRST_SLOT > > > > NIT: align > > align of? This is just a code style thing. It doesn't show here but we usually have 3 spaces between the command the the register, while here there are only 2.
Sorry for the formatting. On Thu, 22 Aug 2019, 23:55 Stefano Stabellini, <sstabellini@kernel.org> wrote: > On Thu, 22 Aug 2019, Julien Grall wrote: > > > > */ > > > > - dsb > > > > + lsr r1, r9, #FIRST_SHIFT > > > > + mov_w r0, LPAE_ENTRY_MASK > > > > > > ldr? > > > > What's wrong with the mov_w? Ok it is two instructions but... the > constant > > will be stored in a literal and therefore induce a memory load (see > patch #8). > > I am just wondering why you would choose mov_w when you can just do a > single simple ldr. > Well, I have just explained it and you likely saw the explanation in patch #8 as you acked it. So I am not sure what other explanation you are looking for. The choice between ldr rX, =... and mov_w is pretty much a matter of taste from our perspective. They will both take 64-bit in memory, the former because of one instruction and the constant stored in the literal pool. The latter because it is using two instructions. Technically, we could also reduce the number of instructions to one for mov_w depending on the constant size. But that's micro-optimization. mov_w will be slightly more efficient because you don't have an extra memory load (from loading from a pool). Although it is pretty much insignificant here. I also have to admit that the syntax for ldr is slightly confusing. I always have to look up in other place how it can be used and works. So I would like to keep the ldr use to the strict minimum, i.e. for non-cons value (such as symbol). Cheers,
diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S index 8f945d318a..34fc9ffcee 100644 --- a/xen/arch/arm/arm32/head.S +++ b/xen/arch/arm/arm32/head.S @@ -32,6 +32,10 @@ #define PT_UPPER(x) (PT_##x & 0xf00) #define PT_LOWER(x) (PT_##x & 0x0ff) +/* Convenience defines to get slot used by Xen mapping. */ +#define XEN_FIRST_SLOT first_table_offset(XEN_VIRT_START) +#define XEN_SECOND_SLOT second_table_offset(XEN_VIRT_START) + #if (defined (CONFIG_EARLY_PRINTK)) && (defined (EARLY_PRINTK_INC)) #include EARLY_PRINTK_INC #endif @@ -158,6 +162,13 @@ past_zImage: ldr r0, =primary_switched mov pc, r0 primary_switched: + /* + * The 1:1 map may clash with other parts of the Xen virtual memory + * layout. As it is not used anymore, remove it completely to + * avoid having to worry about replacing existing mapping + * afterwards. + */ + bl remove_identity_mapping bl setup_fixmap #ifdef CONFIG_EARLY_PRINTK /* Use a virtual address to access the UART. */ @@ -474,12 +485,63 @@ enable_mmu: mov pc, lr ENDPROC(enable_mmu) -setup_fixmap: +/* + * Remove the 1:1 map for the page-tables. It is not easy to keep track + * where the 1:1 map was mapped, so we will look for the top-level entry + * exclusive to the 1:1 map and remove it. + * + * Inputs: + * r9 : paddr(start) + * + * Clobbers r0 - r3 + */ +remove_identity_mapping: + /* r2:r3 := invalid page-table entry */ + mov r2, #0x0 + mov r3, #0x0 /* - * Now we can install the fixmap and dtb mappings, since we - * don't need the 1:1 map any more + * Find the first slot used. Remove the entry for the first + * table if the slot is not XEN_FIRST_SLOT. For slot XEN_FIRST_SLOT, + * the 1:1 mapping was done in the second table. */ - dsb + lsr r1, r9, #FIRST_SHIFT + mov_w r0, LPAE_ENTRY_MASK + and r1, r1, r0 /* r1 := first slot */ + cmp r1, #XEN_FIRST_SLOT + beq 1f + /* It is not in slot 0, remove the entry */ + ldr r0, =boot_pgtable /* r0 := root table */ + lsl r1, r1, #3 /* r1 := Slot offset */ + strd r2, r3, [r0, r1] + b identity_mapping_removed + +1: + /* + * Find the second slot used. Remove the entry for the first + * table if the slot is not XEN_SECOND_SLOT. For slot XEN_SECOND_SLOT, + * it means the 1:1 mapping was not created. + */ + lsr r1, r9, #SECOND_SHIFT + mov_w r0, LPAE_ENTRY_MASK + and r1, r1, r0 /* r1 := second slot */ + cmp r1, #XEN_SECOND_SLOT + beq identity_mapping_removed + /* It is not in slot 1, remove the entry */ + ldr r0, =boot_second /* r0 := second table */ + lsl r1, r1, #3 /* r1 := Slot offset */ + strd r2, r3, [r0, r1] + +identity_mapping_removed: + /* See asm-arm/arm32/flushtlb.h for the explanation of the sequence. */ + dsb nshst + mcr CP32(r0, TLBIALLH) + dsb nsh + isb + + mov pc, lr +ENDPROC(remove_identity_mapping) + +setup_fixmap: #if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */ /* Add UART to the fixmap table */ ldr r1, =xen_fixmap /* r1 := vaddr (xen_fixmap) */ @@ -489,7 +551,6 @@ setup_fixmap: orr r2, r2, #PT_LOWER(DEV_L3) /* r2:r3 := 4K dev map including UART */ mov r3, #0x0 strd r2, r3, [r1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's slot */ -1: /* Map fixmap into boot_second */ ldr r1, =boot_second /* r1 := vaddr (boot_second) */ @@ -501,19 +562,10 @@ setup_fixmap: mov r4, r4, lsr #(SECOND_SHIFT - 3) /* r4 := Slot for FIXMAP(0) */ mov r3, #0x0 strd r2, r3, [r1, r4] /* Map it in the fixmap's slot */ -#endif - - /* - * Flush the TLB in case the 1:1 mapping happens to clash with - * the virtual addresses used by the fixmap or DTB. - */ - dsb /* Ensure any page table updates made above - * have occurred. */ - isb - mcr CP32(r0, TLBIALLH) /* Flush hypervisor TLB */ - dsb /* Ensure completion of TLB flush */ - isb + /* Ensure any page table updates made above have occurred. */ + dsb nshst +#endif mov pc, lr ENDPROC(setup_fixmap)
The 1:1 mapping may clash with other parts of the Xen virtual memory layout. At the moment, Xen is handling the clash by only creating a mapping to the runtime virtual address before enabling the MMU. The rest of the mappings (such as the fixmap) will be mapped after the MMU is enabled. However, the code doing the mapping is not safe as it replace mapping without using the Break-Before-Make sequence. As the 1:1 mapping can be anywhere in the memory, it is easier to remove all the entries added as soon as the 1:1 mapping is not used rather than adding the Break-Before-Make sequence everywhere. It is difficult to track where exactly the 1:1 mapping was created without a full rework of create_page_tables(). Instead, introduce a new function remove_identity_mapping() will look where is the top-level entry for the 1:1 mapping and remove it. The new function is only called for the boot CPU. Secondary CPUs will switch directly to the runtime page-tables so there are no need to remove the 1:1 mapping. Note that this still doesn't make the Secondary CPUs path safe but it is not making it worst. Signed-off-by: Julien Grall <julien.grall@arm.com> --- It is very likely we will need to re-introduce the 1:1 mapping to cater secondary CPUs boot and suspend/resume. For now, the attempt is to make boot CPU path fully Arm Arm compliant. Changes in v3: - Remove unused label - Avoid harcoding slots Changes in v2: - Patch added --- xen/arch/arm/arm32/head.S | 86 +++++++++++++++++++++++++++++++++++++---------- 1 file changed, 69 insertions(+), 17 deletions(-)