Re: [Xen-devel] [PATCH 5/6] x86emul: support INVPCID

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.08.2019 17:31, Andrew Cooper wrote:
> On 01/07/2019 12:57, Jan Beulich wrote:
> > --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> > +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> > @@ -9124,6 +9126,48 @@ x86_emulate(
> >            ASSERT(!state->simd_size);
> >            break;
> >    
> > +    case X86EMUL_OPC_66(0x0f38, 0x82): /* invpcid reg,m128 */
> > +        vcpu_must_have(invpcid);
> > +        generate_exception_if(ea.type != OP_MEM, EXC_UD);
> > +        generate_exception_if(!mode_ring0(), EXC_GP, 0);
> > +
> > +        if ( (rc = ops->read(ea.mem.seg, ea.mem.off, mmvalp, 16,
> > +                             ctxt)) != X86EMUL_OKAY )
> > +            goto done;
> The actual behaviour in hardware is to not even read the memory operand
> if it is unused.  You can demonstrate this by doing an ALL_INC_GLOBAL
> flush with a non-canonical memory operand.
Oh, that's sort of unexpected.

>   In particular, I was
> intending to use this behaviour to speed up handling of INV{EPT,VPID}
> which trap unconditionally.
Which would require the observed behavior to also be the SDM
mandated one, wouldn't it?

> However, this is how the instruction is described in the SDM, and
> INVPCID should usually execute without trapping, so the unconditional
> read should be fine.
>
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Thanks.

Jan

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