| Message ID | 20190709130301.1916-2-linus.walleij@linaro.org (mailing list archive) |
|---|---|
| State | Mainlined |
| Commit | 6583d8298e5981876603b35926b5832f7e99a122 |
| Headers | show |
| Series | [1/2] ARM: boot: Explain the 8 nops | expand |
On Tue, 9 Jul 2019, Linus Walleij wrote: > This open-coded nop as mov r0, r0 is a development history > artifact. > > First commit b11fe38883d1 > ("ARM: 6663/1: make Thumb2 kernel entry point more similar > to the ARM one") moved the code around so that the nops > would come before the conditional thumb instructions, as it > turned out that some boot loaders were patching the initial > nop instructions in the kernel. At this point it is clear > that all mov r0,r0 are open-coded nops. > > Then commit 81a0bc39ea19 ("ARM: add UEFI stub support") > moved things around and defined __nop for EFI support and > missed this open-coded nop. > > commit 06a4b6d009a1 > ("ARM: 8677/1: boot/compressed: fix decompressor header > layout for v7-M") makes all invocations of __nop be wide, > but that is fine, because this is what we want: the > mov r0,r0 is inside ifndef CONFIG_THUMB2_KERNEL. > > Cc: Nicolas Pitre <nico@fluxnic.net> > Cc: Roy Franz <roy.franz@cavium.com> > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> > Signed-off-by: Linus Walleij <linus.walleij@linaro.org> For both patches: Acked-by: Nicolas Pitre <nico@fluxnic.net> > --- > arch/arm/boot/compressed/head.S | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S > index 544450c90673..93dffed0ac6e 100644 > --- a/arch/arm/boot/compressed/head.S > +++ b/arch/arm/boot/compressed/head.S > @@ -169,7 +169,7 @@ start: > __nop > .endr > #ifndef CONFIG_THUMB2_KERNEL > - mov r0, r0 > + __nop > #else > AR_CLASS( sub pc, pc, #3 ) @ A/R: switch to Thumb2 mode > M_CLASS( nop.w ) @ M: already in Thumb2 mode > -- > 2.21.0 > >
On 7/9/19 6:03 AM, Linus Walleij wrote: > External Email > > ---------------------------------------------------------------------- > This open-coded nop as mov r0, r0 is a development history > artifact. > > First commit b11fe38883d1 > ("ARM: 6663/1: make Thumb2 kernel entry point more similar > to the ARM one") moved the code around so that the nops > would come before the conditional thumb instructions, as it > turned out that some boot loaders were patching the initial > nop instructions in the kernel. At this point it is clear > that all mov r0,r0 are open-coded nops. > > Then commit 81a0bc39ea19 ("ARM: add UEFI stub support") > moved things around and defined __nop for EFI support and > missed this open-coded nop. > > commit 06a4b6d009a1 > ("ARM: 8677/1: boot/compressed: fix decompressor header > layout for v7-M") makes all invocations of __nop be wide, > but that is fine, because this is what we want: the > mov r0,r0 is inside ifndef CONFIG_THUMB2_KERNEL. > > Cc: Nicolas Pitre <nico@fluxnic.net> > Cc: Roy Franz <roy.franz@cavium.com> > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> > Signed-off-by: Linus Walleij <linus.walleij@linaro.org> For both patches Acked-by: Roy Franz <rfranz@marvell.com> > --- > arch/arm/boot/compressed/head.S | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S > index 544450c90673..93dffed0ac6e 100644 > --- a/arch/arm/boot/compressed/head.S > +++ b/arch/arm/boot/compressed/head.S > @@ -169,7 +169,7 @@ start: > __nop > .endr > #ifndef CONFIG_THUMB2_KERNEL > - mov r0, r0 > + __nop > #else > AR_CLASS( sub pc, pc, #3 ) @ A/R: switch to Thumb2 mode > M_CLASS( nop.w ) @ M: already in Thumb2 mode >
diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S index 544450c90673..93dffed0ac6e 100644 --- a/arch/arm/boot/compressed/head.S +++ b/arch/arm/boot/compressed/head.S @@ -169,7 +169,7 @@ start: __nop .endr #ifndef CONFIG_THUMB2_KERNEL - mov r0, r0 + __nop #else AR_CLASS( sub pc, pc, #3 ) @ A/R: switch to Thumb2 mode M_CLASS( nop.w ) @ M: already in Thumb2 mode
This open-coded nop as mov r0, r0 is a development history artifact. First commit b11fe38883d1 ("ARM: 6663/1: make Thumb2 kernel entry point more similar to the ARM one") moved the code around so that the nops would come before the conditional thumb instructions, as it turned out that some boot loaders were patching the initial nop instructions in the kernel. At this point it is clear that all mov r0,r0 are open-coded nops. Then commit 81a0bc39ea19 ("ARM: add UEFI stub support") moved things around and defined __nop for EFI support and missed this open-coded nop. commit 06a4b6d009a1 ("ARM: 8677/1: boot/compressed: fix decompressor header layout for v7-M") makes all invocations of __nop be wide, but that is fine, because this is what we want: the mov r0,r0 is inside ifndef CONFIG_THUMB2_KERNEL. Cc: Nicolas Pitre <nico@fluxnic.net> Cc: Roy Franz <roy.franz@cavium.com> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> Signed-off-by: Linus Walleij <linus.walleij@linaro.org> --- arch/arm/boot/compressed/head.S | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)