Re: [PATCH v5] x86/entry_32: Use stack segment selector for VERW operand

[Pawan Gupta]

On Thu, Aug 29, 2024 at 03:28:13PM -0700, Dave Hansen wrote:
> On 7/11/24 15:03, Pawan Gupta wrote:
> > +/*
> > + * Safer version of CLEAR_CPU_BUFFERS that uses %ss to reference VERW operand
> > + * mds_verw_sel. This ensures VERW will not #GP for an arbitrary user %ds.
> > + */
> > +.macro CLEAR_CPU_BUFFERS_SAFE
> > +	ALTERNATIVE "", __stringify(verw %ss:_ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
> > +.endm
> 
> One other thing...
> 
> Instead of making a "_SAFE" variant, let's just make the 32-bit version
> always safe.

That sounds good to me.

> Also, is there any downside to using %ss: on 64-bit?  If not, let's just
> update the one and only CLEAR_CPU_BUFFERS use %ss:.

A quick test after adding %ss: on 64-bit doesn't show any significant
latency difference. I will revise the patch.