Re: [Xen-devel] [PATCH 2/3] x86/HVM: support (emulate) UMIP

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/06/2016 06:44 AM, Jan Beulich wrote:
> There are three noteworthy drawbacks:
> 1) The intercepts we need to enable here are CPL-independent, i.e. we
>    now have to emulate certain instructions for ring 0.
> 2) On VMX there's no intercept for SMSW, so the emulation isn't really
>    complete there.
> 3) The CR4 write intercept on SVM is lower priority than all exception
>    checks, so we need to intercept #GP.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> The tool stack change could be left out - it updates a table which is
> rather out of date anyway.
> ---
> This once again points out that handle_mmio() is rather badly named, as
> it's about more than just MMIO. Since we have hvm_emulate_one()
> already, I am, however, lacking an idea for a good alternative name.
>
> --- a/tools/libxl/libxl_cpuid.c
> +++ b/tools/libxl/libxl_cpuid.c
> @@ -158,6 +158,7 @@ int libxl_cpuid_parse_config(libxl_cpuid
>          {"de",           0x00000001, NA, CPUID_REG_EDX,  2,  1},
>          {"vme",          0x00000001, NA, CPUID_REG_EDX,  1,  1},
>          {"fpu",          0x00000001, NA, CPUID_REG_EDX,  0,  1},
> +        {"umip",         0x00000007,  0, CPUID_REG_ECX,  2,  1},
>          {"topoext",      0x80000001, NA, CPUID_REG_ECX, 22,  1},
>          {"tbm",          0x80000001, NA, CPUID_REG_ECX, 21,  1},
>          {"nodeid",       0x80000001, NA, CPUID_REG_ECX, 19,  1},
> --- a/xen/arch/x86/cpuid.c
> +++ b/xen/arch/x86/cpuid.c
> @@ -154,6 +154,13 @@ static void __init calculate_hvm_feature
>      __set_bit(X86_FEATURE_APIC, hvm_featureset);
>  
>      /*
> +     * Xen can often provide UMIP emulation to HVM guests even if the host
> +     * doesn't have such functionality.
> +     */
> +    if ( cpu_has_vmx_dt_exiting || cpu_has_svm )
> +        __set_bit(X86_FEATURE_UMIP, hvm_featureset);

I don't think I understand how this is going to work for processors that
don't support UMIP.

How, for example, can guest_cr[4] have X86_CR4_UMIP set on these
processors when CPUID will not show the feature being there?

-boris


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