Re: [Xen-devel] [PATCH 8/8] x86/emul: Implement the STAC and CLAC instructions

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.12.16 at 11:09, <andrew.cooper3@xxxxxxxxxx> wrote:
> @@ -4362,11 +4363,27 @@ x86_emulate(
>  
>          switch( modrm )
>          {
> -#ifdef __XEN__
> -        case 0xd1: /* xsetbv */
> -        {
>              unsigned long cr4;
>  
> +        case 0xca: /* clac */
> +        case 0xcb: /* stac */
> +            generate_exception_if(
> +                lock_prefix || (_regs.eflags & EFLG_VM), EXC_UD);
> +            if ( !ops->read_cr || ops->read_cr(4, &cr4, ctxt) != 
> X86EMUL_OKAY )
> +                cr4 = 0;
> +            /*
> +             * Contrary to expectation (i.e. #GP[0]), #UD for the CPL check 
> is
> +             * the documented and observed behaviour.
> +             */
> +            generate_exception_if(!(cr4 & CR4_SMAP) || !mode_ring0(), 
> EXC_UD);

If documentation is to be trusted, then there's no CR4.SMAP check
supposed to be here, but just a CPUID one.

Otoh I assume documentation can't be trusted regarding the use of
prefixes 66, F2, and F3: Just like they're apparently illegal to use with
VMFUNC (thread still pending with Intel) and like documented for e.g.
XGETBV and XSETBV, I would think you need a vex.pfx check here
despite the SDM not explicitly saying so.

Jan


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