Re: [Patch] KVM: SVM: Fix svm_xsaves_supported

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Jim Mattson <jmattson@google.com> writes:

> On Wed, Sep 4, 2019 at 9:51 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
>> Currently, VMX code only supports writing '0' to MSR_IA32_XSS:
>>
>>         case MSR_IA32_XSS:
>>                 if (!vmx_xsaves_supported() ||
>>                     (!msr_info->host_initiated &&
>>                      !(guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
>>                        guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))))
>>                         return 1;
>>                 /*
>>                  * The only supported bit as of Skylake is bit 8, but
>>                  * it is not supported on KVM.
>>                  */
>>                 if (data != 0)
>>                         return 1;
>>
>>
>> we will probably need the same limitation for SVM, however, I'd vote for
>> creating separate kvm_x86_ops->set_xss() implementations.
>
> I hope separate implementations are unnecessary. The allowed IA32_XSS
> bits should be derivable from guest_cpuid_has() in a
> vendor-independent way. Otherwise, the CPU vendors have messed up. :-)
>
> At present, we use the MSR-load area to swap guest/host values of
> IA32_XSS on Intel (when the host and guest values differ), but it
> seems to me that IA32_XSS and %xcr0 should be swapped at the same
> time, in kvm_load_guest_xcr0/kvm_put_guest_xcr0. This potentially adds
> an additional L1 WRMSR VM-exit to every emulated VM-entry or VM-exit
> for nVMX, but since the host currently sets IA32_XSS to 0 and we only
> allow the guest to set IA32_XSS to 0, we can probably worry about this
> later.

Yea, I was suggesting to split implementations as a future proof but a
comment like "This ought to be 0 for now" would also do)

>
> I have to say, this is an awful lot of effort for an MSR that's never used!

Agreed :-)

-- 
Vitaly