| Message ID | 1462815566-15086-1-git-send-email-daniel.thompson@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Hi Daniel, So does kgdb do something useful after this patch? On Mon, May 09, 2016 at 06:39:26PM +0100, Daniel Thompson wrote: > Current versions of gdb do not interoperate cleanly with kgdb on arm64 > systems because gdb and kgdb do not use the same register description. > This patch modifies kgdb to work with recent releases of gdb (>= 7.8.1). > > Compatibility with gdb (after the patch is applied) is as follows: > > gdb-7.6 and earlier Ok > gdb-7.7 series Works if user provides custom target description > gdb-7.8(.0) Works if user provides custom target description > gdb-7.8.1 and later Ok > > When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was > introduced it was paired with a gdb patch that made an incompatible > change to the gdbserver protocol. This patch was eventually merged into > the gdb sources: > https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a4d9ba85ec5597a6a556afe26b712e878374b9dd > > The change to the protocol was mostly made to simplify big-endian support > inside the kernel gdb stub. Unfortunately the gdb project released > gdb-7.7.x and gdb-7.8.0 before the protocol incompatibility was identified > and reversed: > https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=bdc144174bcb11e808b4e73089b850cf9620a7ee > > This leaves us in a position where kgdb still uses the no-longer-used > protocol; gdb-7.8.1, which restored the original behaviour,If was > released on 2014-10-29. > > I don't believe it is possible to detect/correct the protocol > incompatiblity which means the kernel must take a view about which > version of the gdb remote protocol is "correct". This patch takes the > view that the original/current version of the protocol is correct > and that version found in gdb-7.7.x and gdb-7.8.0 is anomalous. Urgh, this is filthy! Still, without a time machine, I guess there's little we can do about it. Can I ask you to respin the patch but with the rationale as a comment in the header file, and a pointer to the comment from the C code too, please? The code looks incorrect after this change, so we should justify how we've ended up in this state and not everybody looks at the git log for that rationale. Cheers, Will
On Mon, May 09, 2016 at 06:39:26PM +0100, Daniel Thompson wrote: > Current versions of gdb do not interoperate cleanly with kgdb on arm64 > systems because gdb and kgdb do not use the same register description. > This patch modifies kgdb to work with recent releases of gdb (>= 7.8.1). > > Compatibility with gdb (after the patch is applied) is as follows: > > gdb-7.6 and earlier Ok > gdb-7.7 series Works if user provides custom target description > gdb-7.8(.0) Works if user provides custom target description > gdb-7.8.1 and later Ok > > When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was > introduced it was paired with a gdb patch that made an incompatible > change to the gdbserver protocol. This patch was eventually merged into > the gdb sources: > https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a4d9ba85ec5597a6a556afe26b712e878374b9dd > > The change to the protocol was mostly made to simplify big-endian support > inside the kernel gdb stub. While that was how we discovered the inconsistency, a major concern is that SPSR_EL* (i.e. PSTATE), as accessed by MRS/MSR is a 64-bit quantity, even if the upper 32 bits are RES0 today. It is conceivable that the upper 32 bits could be used in future (as happened with CLIDR_EL1), and for this reason we expose those upper 32 bits from the kernel, and treat system registers as 64-bit quantities generally. So this was also about ensuring the interface was consistent and to some extent future-proof. Thanks, Mark.
On 10/05/16 11:45, Mark Rutland wrote: >> When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was >> introduced it was paired with a gdb patch that made an incompatible >> change to the gdbserver protocol. This patch was eventually merged into >> the gdb sources: >> https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a4d9ba85ec5597a6a556afe26b712e878374b9dd >> >> The change to the protocol was mostly made to simplify big-endian support >> inside the kernel gdb stub. > > While that was how we discovered the inconsistency, a major concern is > that SPSR_EL* (i.e. PSTATE), as accessed by MRS/MSR is a 64-bit > quantity, even if the upper 32 bits are RES0 today. > > It is conceivable that the upper 32 bits could be used in future (as > happened with CLIDR_EL1), and for this reason we expose those upper 32 > bits from the kernel, and treat system registers as 64-bit quantities > generally. These cases are not exactly the same. CLIDR_ELx is (or was) architecturally defined as a 64-bit register and explicitly marks the upper 32-bits as RES0. That is not the case for SPSR_ELx; this register is architecturally defined to be 32-bit. I know that doesn't *prevent* SPSR_ELx from being expanded in the future it is not unreasonable for gdb to design its wire protocol based on the description found in the architecture manual. > So this was also about ensuring the interface was consistent and to some > extent future-proof. gdb remote protocol is already future proof and has never at any point contradicted the architecture. However the changes to the protocol in 7.7.x and 7.8.0 were analogous to an unexpected ABI change rather than a carefully controlled introduction of a new feature. Like the kernel, once detected, they were reversed ;-) . Daniel.
On 10/05/16 11:28, Will Deacon wrote: > Hi Daniel, > > So does kgdb do something useful after this patch? On this occasion I was debugging it on behalf of another developer rather than trying to use it myself to debug something so I didn't do much additional testing beyond sanity tested the contents of the register set. Naturally it will be *totally* awesome if I get the pseudo-NMI stuff working perfectly. ;-) > On Mon, May 09, 2016 at 06:39:26PM +0100, Daniel Thompson wrote: >> Current versions of gdb do not interoperate cleanly with kgdb on arm64 >> systems because gdb and kgdb do not use the same register description. >> This patch modifies kgdb to work with recent releases of gdb (>= 7.8.1). >> >> Compatibility with gdb (after the patch is applied) is as follows: >> >> gdb-7.6 and earlier Ok >> gdb-7.7 series Works if user provides custom target description >> gdb-7.8(.0) Works if user provides custom target description >> gdb-7.8.1 and later Ok >> >> When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was >> introduced it was paired with a gdb patch that made an incompatible >> change to the gdbserver protocol. This patch was eventually merged into >> the gdb sources: >> https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a4d9ba85ec5597a6a556afe26b712e878374b9dd >> >> The change to the protocol was mostly made to simplify big-endian support >> inside the kernel gdb stub. Unfortunately the gdb project released >> gdb-7.7.x and gdb-7.8.0 before the protocol incompatibility was identified >> and reversed: >> https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=bdc144174bcb11e808b4e73089b850cf9620a7ee >> >> This leaves us in a position where kgdb still uses the no-longer-used >> protocol; gdb-7.8.1, which restored the original behaviour,If was >> released on 2014-10-29. >> >> I don't believe it is possible to detect/correct the protocol >> incompatiblity which means the kernel must take a view about which >> version of the gdb remote protocol is "correct". This patch takes the >> view that the original/current version of the protocol is correct >> and that version found in gdb-7.7.x and gdb-7.8.0 is anomalous. > > Urgh, this is filthy! Still, without a time machine, I guess there's > little we can do about it. Can I ask you to respin the patch but with > the rationale as a comment in the header file, and a pointer to the > comment from the C code too, please? > > The code looks incorrect after this change, so we should justify how > we've ended up in this state and not everybody looks at the git log > for that rationale. Will do. Daniel.
On Tue, May 10, 2016 at 02:41:54PM +0100, Daniel Thompson wrote: > On 10/05/16 11:45, Mark Rutland wrote: > >>When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was > >>introduced it was paired with a gdb patch that made an incompatible > >>change to the gdbserver protocol. This patch was eventually merged into > >>the gdb sources: > >>https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a4d9ba85ec5597a6a556afe26b712e878374b9dd > >> > >>The change to the protocol was mostly made to simplify big-endian support > >>inside the kernel gdb stub. > > > >While that was how we discovered the inconsistency, a major concern is > >that SPSR_EL* (i.e. PSTATE), as accessed by MRS/MSR is a 64-bit > >quantity, even if the upper 32 bits are RES0 today. > > > >It is conceivable that the upper 32 bits could be used in future (as > >happened with CLIDR_EL1), and for this reason we expose those upper 32 > >bits from the kernel, and treat system registers as 64-bit quantities > >generally. > > These cases are not exactly the same. > > CLIDR_ELx is (or was) architecturally defined as a 64-bit register > and explicitly marks the upper 32-bits as RES0. > > That is not the case for SPSR_ELx; this register is architecturally > defined to be 32-bit. The below doesn't necessarily change your subsequent argument, but that isn't quite true. The two cases are in fact identical if you dig into the history a bit further. Take a look in an earlier revision of the ARM ARM (e.g. ARM DDI 0487A.b), where it was explicitly stated: Attributes CLIDR_EL1 is a 32-bit register. This was subsequently upgraded to 64-bit with the addition of ICB. Based on this, my PoV is that any register that the ARM ARM describes as "a 32-bit register" is a 64-bit register for which the upper 32 bits are RES0. > I know that doesn't *prevent* SPSR_ELx from being expanded in the > future it is not unreasonable for gdb to design its wire protocol > based on the description found in the architecture manual. > > >So this was also about ensuring the interface was consistent and to some > >extent future-proof. > > gdb remote protocol is already future proof and has never at any > point contradicted the architecture. > > However the changes to the protocol in 7.7.x and 7.8.0 were > analogous to an unexpected ABI change rather than a carefully > controlled introduction of a new feature. Like the kernel, once > detected, they were reversed ;-) . To be clear, I don't disagree with this. The ARM ARM is at best amigibious w.r.t. what it means by "a 32-bit register", and that's the only point of contention. However, we should make note of the above as a key point of rationale, as it affects other decisions we make in this area. Thanks, Mark.
On 10/05/16 15:31, Mark Rutland wrote: > On Tue, May 10, 2016 at 02:41:54PM +0100, Daniel Thompson wrote: >> On 10/05/16 11:45, Mark Rutland wrote: >>>> When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was >>>> introduced it was paired with a gdb patch that made an incompatible >>>> change to the gdbserver protocol. This patch was eventually merged into >>>> the gdb sources: >>>> https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a4d9ba85ec5597a6a556afe26b712e878374b9dd >>>> >>>> The change to the protocol was mostly made to simplify big-endian support >>>> inside the kernel gdb stub. >>> >>> While that was how we discovered the inconsistency, a major concern is >>> that SPSR_EL* (i.e. PSTATE), as accessed by MRS/MSR is a 64-bit >>> quantity, even if the upper 32 bits are RES0 today. >>> >>> It is conceivable that the upper 32 bits could be used in future (as >>> happened with CLIDR_EL1), and for this reason we expose those upper 32 >>> bits from the kernel, and treat system registers as 64-bit quantities >>> generally. >> >> These cases are not exactly the same. >> >> CLIDR_ELx is (or was) architecturally defined as a 64-bit register >> and explicitly marks the upper 32-bits as RES0. >> >> That is not the case for SPSR_ELx; this register is architecturally >> defined to be 32-bit. > > The below doesn't necessarily change your subsequent argument, but that > isn't quite true. > > The two cases are in fact identical if you dig into the history a bit > further. Take a look in an earlier revision of the ARM ARM (e.g. ARM DDI > 0487A.b), where it was explicitly stated: > > Attributes > CLIDR_EL1 is a 32-bit register. I see. I only when back as far as A.e ... > Based on this, my PoV is that any register that the ARM ARM describes as > "a 32-bit register" is a 64-bit register for which the upper 32 bits are > RES0. Understood. >> I know that doesn't *prevent* SPSR_ELx from being expanded in the >> future it is not unreasonable for gdb to design its wire protocol >> based on the description found in the architecture manual. >> >>> So this was also about ensuring the interface was consistent and to some >>> extent future-proof. >> >> gdb remote protocol is already future proof and has never at any >> point contradicted the architecture. >> >> However the changes to the protocol in 7.7.x and 7.8.0 were >> analogous to an unexpected ABI change rather than a carefully >> controlled introduction of a new feature. Like the kernel, once >> detected, they were reversed ;-) . > > To be clear, I don't disagree with this. The ARM ARM is at best > amigibious w.r.t. what it means by "a 32-bit register", and that's the > only point of contention. Agree... such ambiguity strikes me as a particularly serious problem for switchable context registers since it takes *very* close attention to the ARM ARM for an existing OS to determine that it is required to treat the upper 32-bits of context should-be-zero-or-preserved. > However, we should make note of the above as a key point of rationale, > as it affects other decisions we make in this area. Ok. I'll add comments describing how the gdbremote protocol is interpreting the spec. Daniel.
diff --git a/arch/arm64/include/asm/kgdb.h b/arch/arm64/include/asm/kgdb.h index f69f69c8120c..326fa8f44da5 100644 --- a/arch/arm64/include/asm/kgdb.h +++ b/arch/arm64/include/asm/kgdb.h @@ -43,20 +43,17 @@ extern int kgdb_fault_expected; * General purpose regs: * r0-r30: 64 bit * sp,pc : 64 bit - * pstate : 64 bit - * Total: 34 + * pstate : 32 bit + * Total: 33 + 1 * FPU regs: * f0-f31: 128 bit - * Total: 32 - * Extra regs * fpsr & fpcr: 32 bit - * Total: 2 - * + * Total: 32 + 2 */ -#define _GP_REGS 34 +#define _GP_REGS 33 #define _FP_REGS 32 -#define _EXTRA_REGS 2 +#define _EXTRA_REGS 3 /* * general purpose registers size in bytes. * pstate is only 4 bytes. subtract 4 bytes diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c index b67531a13136..ec62a4e3c190 100644 --- a/arch/arm64/kernel/kgdb.c +++ b/arch/arm64/kernel/kgdb.c @@ -58,7 +58,11 @@ struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = { { "x30", 8, offsetof(struct pt_regs, regs[30])}, { "sp", 8, offsetof(struct pt_regs, sp)}, { "pc", 8, offsetof(struct pt_regs, pc)}, - { "pstate", 8, offsetof(struct pt_regs, pstate)}, + { "pstate", 4, offsetof(struct pt_regs, pstate) +#ifdef CONFIG_CPU_BIG_ENDIAN + + 4 +#endif + }, { "v0", 16, -1 }, { "v1", 16, -1 }, { "v2", 16, -1 }, @@ -128,6 +132,7 @@ sleeping_thread_to_gdb_regs(unsigned long *gdb_regs, struct task_struct *task) memset((char *)gdb_regs, 0, NUMREGBYTES); thread_regs = task_pt_regs(task); memcpy((void *)gdb_regs, (void *)thread_regs->regs, GP_REG_BYTES); + dbg_get_reg(33, gdb_regs + GP_REG_BYTES, thread_regs); } void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long pc)
Current versions of gdb do not interoperate cleanly with kgdb on arm64 systems because gdb and kgdb do not use the same register description. This patch modifies kgdb to work with recent releases of gdb (>= 7.8.1). Compatibility with gdb (after the patch is applied) is as follows: gdb-7.6 and earlier Ok gdb-7.7 series Works if user provides custom target description gdb-7.8(.0) Works if user provides custom target description gdb-7.8.1 and later Ok When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was introduced it was paired with a gdb patch that made an incompatible change to the gdbserver protocol. This patch was eventually merged into the gdb sources: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a4d9ba85ec5597a6a556afe26b712e878374b9dd The change to the protocol was mostly made to simplify big-endian support inside the kernel gdb stub. Unfortunately the gdb project released gdb-7.7.x and gdb-7.8.0 before the protocol incompatibility was identified and reversed: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=bdc144174bcb11e808b4e73089b850cf9620a7ee This leaves us in a position where kgdb still uses the no-longer-used protocol; gdb-7.8.1, which restored the original behaviour,If was released on 2014-10-29. I don't believe it is possible to detect/correct the protocol incompatiblity which means the kernel must take a view about which version of the gdb remote protocol is "correct". This patch takes the view that the original/current version of the protocol is correct and that version found in gdb-7.7.x and gdb-7.8.0 is anomalous. Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org> --- Notes: Vijaya: I really hope I have the history lesson in the commit message correct! I've done a fair bit of archeology to unpick things. arch/arm64/include/asm/kgdb.h | 13 +++++-------- arch/arm64/kernel/kgdb.c | 7 ++++++- 2 files changed, 11 insertions(+), 9 deletions(-) -- 2.5.5