On Mon, Apr 27, 2015 at 07:57:36AM -0700, Linus Torvalds wrote:
> On Mon, Apr 27, 2015 at 4:35 AM, Borislav Petkov <b...@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?
> In fact, aim to make the conditional jump be a
> two-byte one (jump forward to another jump if required - it's a
> slow-path that doesn't matter at *all* for the taken case), and the
> end result is just six bytes. That way you can use alternative to
> replace it with one single noop on AMD.
Well, even with the non-optimal NOPs (we end up with 4 3-byte NOPs and
one single-byte), we're still better than the unconditional JMP I had
there before:
https://lkml.kernel.org/r/20150427143905.gk6...@pd.tnic
(you might want to look at the raw email - marc.info breaks lines)
I'll retest with the F16h NOPs to see whether there's any difference.
Thanks.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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