Re: [Xen-devel] [PATCH 1/3] x86emul: honor guest CR4.OSFXSR, CR4.OSXSAVE, and CR0.PE/EFLAGS.VM

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/10/16 14:39, Jan Beulich wrote:
> @@ -770,9 +773,23 @@ static int _get_fpu(
>          unsigned long cr0;
>  
>          fail_if(!ops->read_cr);
> +        if ( type >= X86EMUL_FPU_xmm )
> +        {
> +            unsigned long cr4;
> +
> +            rc = ops->read_cr(4, &cr4, ctxt);
> +            if ( rc != X86EMUL_OKAY )
> +                return rc;
> +            generate_exception_if(!(cr4 & ((type == X86EMUL_FPU_xmm)
> +                                           ? CR4_OSFXSR : CR4_OSXSAVE)),
> +                                  EXC_UD, -1);
> +        }
> +
>          rc = ops->read_cr(0, &cr0, ctxt);
>          if ( rc != X86EMUL_OKAY )
>              return rc;
> +        if ( !(cr0 & CR0_PE) || (ctxt->regs->eflags & EFLG_VM) )
> +            generate_exception_if(type >= X86EMUL_FPU_ymm, EXC_UD, -1);

Is this an appropriate check to do here?  This restriction is because
the VEX prefix isn't permitted in real/vm86 mode.

Instead of a generate_exception_if(), I would instead have an ASSERT()
that we don't actually reach this point.

~Andrew

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