Messages in this thread Patch in this message | | | Date | Wed, 8 Feb 2023 21:36:24 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 3/4] x86/alternative: Rewrite optimize_nops() some |
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On Wed, Feb 08, 2023 at 09:29:23PM +0100, Peter Zijlstra wrote:
> As is, there were only a hand full of NOPs that turned into this jmp.d8 > thing on the defconfig+kvm_guest.config I build to test this -- it is by > no means a common thing. And about half of them would be gone by > extending the max nop length to at least 10 or so. > > In fact, I did that patch once, lemme see if I still have it...
Even still applies too, lemme go test that.
Also, there's a bunch of alternatives() where we explicitly put in that short jmp instead of taking the nops, see for example:
$ git grep -i "alternative.*jmp" arch/x86/
--- Subject: x86_64: Longer NOPs From: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- arch/x86/include/asm/nops.h | 13 +++++++++++-- arch/x86/kernel/alternative.c | 8 ++++++++ 2 files changed, 19 insertions(+), 2 deletions(-)
--- a/arch/x86/include/asm/nops.h +++ b/arch/x86/include/asm/nops.h @@ -34,6 +34,8 @@ #define BYTES_NOP7 0x8d,0xb4,0x26,0x00,0x00,0x00,0x00 #define BYTES_NOP8 0x3e,BYTES_NOP7 +#define ASM_NOP_MAX 8 + #else /* @@ -47,6 +49,8 @@ * 6: osp nopl 0x00(%eax,%eax,1) * 7: nopl 0x00000000(%eax) * 8: nopl 0x00000000(%eax,%eax,1) + * 9: cs nopl 0x00000000(%eax,%eax,1) + * 10: osp cs nopl 0x00000000(%eax,%eax,1) */ #define BYTES_NOP1 0x90 #define BYTES_NOP2 0x66,BYTES_NOP1 @@ -56,6 +60,13 @@ #define BYTES_NOP6 0x66,BYTES_NOP5 #define BYTES_NOP7 0x0f,0x1f,0x80,0x00,0x00,0x00,0x00 #define BYTES_NOP8 0x0f,0x1f,0x84,0x00,0x00,0x00,0x00,0x00 +#define BYTES_NOP9 0x2e,BYTES_NOP8 +#define BYTES_NOP10 0x66,BYTES_NOP9 + +#define ASM_NOP9 _ASM_BYTES(BYTES_NOP9) +#define ASM_NOP10 _ASM_BYTES(BYTES_NOP10) + +#define ASM_NOP_MAX 10 #endif /* CONFIG_64BIT */ @@ -68,8 +79,6 @@ #define ASM_NOP7 _ASM_BYTES(BYTES_NOP7) #define ASM_NOP8 _ASM_BYTES(BYTES_NOP8) -#define ASM_NOP_MAX 8 - #ifndef __ASSEMBLY__ extern const unsigned char * const x86_nops[]; #endif --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -86,6 +86,10 @@ static const unsigned char x86nops[] = BYTES_NOP6, BYTES_NOP7, BYTES_NOP8, +#ifdef CONFIG_64BIT + BYTES_NOP9, + BYTES_NOP10, +#endif }; const unsigned char * const x86_nops[ASM_NOP_MAX+1] = @@ -99,6 +103,10 @@ const unsigned char * const x86_nops[ASM x86nops + 1 + 2 + 3 + 4 + 5, x86nops + 1 + 2 + 3 + 4 + 5 + 6, x86nops + 1 + 2 + 3 + 4 + 5 + 6 + 7, +#ifdef CONFIG_64BIT + x86nops + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8, + x86nops + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9, +#endif }; /* Use this to add nops to a buffer, then text_poke the whole buffer. */
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