Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 27 Apr 2015 08:56:22 -0700 | Subject | Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue |
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On Mon, Apr 27, 2015 at 8:46 AM, Borislav Petkov <bp@alien8.de> wrote: > On Mon, Apr 27, 2015 at 07:57:36AM -0700, 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. > > Right, what about the false positives: > > 17be0aec74fb ("x86/asm/entry/64: Implement better check for canonical addresses") > > ? We don't care?
The false positives only matter for very strange workloads, e.g. vsyscall=native with old libc. If it's a measurable regression, we could revert it.
--Andy
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