Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

Pawan Gupta
2023-10-21T02:21:50Z

On Sat, Oct 21, 2023 at 02:33:47AM +0100, Andrew Cooper wrote:
> On 21/10/2023 2:18 am, Pawan Gupta wrote:
> > On Sat, Oct 21, 2023 at 12:55:45AM +0100, Andrew Cooper wrote:
> >> Also it avoids playing games with hiding data inside an instruction.
> >> It's a neat trick, but the neater trick is avoid it whenever possible.
> > Thanks for the pointers. I think verw in 32-bit mode won't be able to
> > address the operand outside of 4GB range.
>
> And?  In a 32bit kernel, what lives outside of a 4G range?
>
> > Maybe this is fine or could it
> > be a problem addressing from e.g. KVM module?
>
> RIP-relative addressing is disp32.  Which is the same as it is for
> direct calls.
>
> So if your module is far enough away for VERW to have issues, you've got
> far more basic problems to solve first.

Sorry, I raised the wrong problem. In 64-bit mode, verww only has 32-bit
of relative addressing, so memory operand has to be within 4GB of
callsite. That could be a constraint.