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

Denys Vlasenko
2015-04-27T16:12:40Z

On 04/27/2015 04:57 PM, Linus Torvalds wrote:
> On Mon, Apr 27, 2015 at 4:35 AM, Borislav Petkov <bp@alien8.de> 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)