Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

[Denys Vlasenko]

On 04/27/2015 04:57 PM, Linus Torvalds wrote:
> On Mon, Apr 27, 2015 at 4:35 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
>>
>>         /*
>>          * Change top 16 bits to be the sign-extension of 47th bit, if this
>>          * changed %rcx, it was not canonical.
>>          */
>>         ALTERNATIVE "", \
>>                 "shl    $(64 - (47+1)), %rcx; \
>>                  sar    $(64 - (47+1)), %rcx; \
>>                  cmpq   %rcx, %r11; \
>>                  jne    opportunistic_sysret_failed", X86_BUG_SYSRET_CANON_RCX
> 
> Guys, if we're looking at cycles for this, then don't do the "exact
> canonical test". and go back to just doing
> 
>         shr $__VIRTUAL_MASK_SHIFT, %rcx
>         jnz opportunistic_sysret_failed
> 
> which is much smaller.

It is smaller, but not by much. It is two instructions smaller.
On disassembly level, the changes are:

cmp %rcx,0x80(%rsp) -> mov 0x80(%rsp),%r11; cmp %rcx,%r11
shr $0x2f,%rcx      -> shl $0x10,%rcx; sar $0x10,%rcx; cmp %rcx,%r11
mov 0x58(%rsp),%rcx -> (eliminated)


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