Re: [Xen-devel] [PATCH v2 3/3] x86/emul: Support CPUID faulting via a speculative MSR read

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.02.17 at 11:28, <andrew.cooper3@xxxxxxxxxx> wrote:
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -5424,10 +5424,25 @@ x86_emulate(
>          break;
>  
>      case X86EMUL_OPC(0x0f, 0xa2): /* cpuid */
> +        msr_val = 0;
>          fail_if(ops->cpuid == NULL);
> +
> +        /* Speculatively read MSR_INTEL_MISC_FEATURES_ENABLES. */
> +        if ( ops->read_msr &&
> +             (rc = ops->read_msr(MSR_INTEL_MISC_FEATURES_ENABLES,
> +                                 &msr_val, ctxt)) == X86EMUL_EXCEPTION )
> +        {
> +            /* Not implemented.  Squash the exception and proceed normally. 
> */
> +            x86_emul_reset_event(ctxt);
> +            rc = X86EMUL_OKAY;
> +        }
> +        if ( rc != X86EMUL_OKAY )
> +            goto done;
> +
> +        generate_exception_if((msr_val & MSR_MISC_FEATURES_CPUID_FAULTING) &&
> +                              !mode_ring0(), EXC_GP, 0); /* Faulting active? 
> */

Could you please move the mode_ring0() check up to bypass the
MSR read in the first place?

> --- a/xen/arch/x86/x86_emulate/x86_emulate.h
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.h
> @@ -413,12 +413,7 @@ struct x86_emulate_ops
>      int (*wbinvd)(
>          struct x86_emulate_ctxt *ctxt);
>  
> -    /*
> -     * cpuid: Emulate CPUID via given set of EAX-EDX inputs/outputs.
> -     *
> -     * May return X86EMUL_EXCEPTION, which causes the emulator to inject
> -     * #GP[0].  Used to implement CPUID faulting.
> -     */
> +    /* cpuid: Emulate CPUID via given set of EAX-EDX inputs/outputs. */
>      int (*cpuid)(
>          uint32_t leaf,
>          uint32_t subleaf,

Are there any ways left for the hook to fail? IOW, should its return
type become void now?

Jan


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