Re: [PATCH] KVM: vmx: Fix the broken usage of vmx_xsaves_supported

[Xiaoyao Li]

On 6/20/2019 2:40 PM, Wanpeng Li wrote:
> Hi,
> On Thu, 20 Jun 2019 at 13:06, Tao Xu <tao3.xu@xxxxxxxxx> wrote:
> > The helper vmx_xsaves_supported() returns the bit value of
> > SECONDARY_EXEC_XSAVES in vmcs_config.cpu_based_2nd_exec_ctrl, which
> > remains unchanged true if vmcs supports 1-setting of this bit after
> > setup_vmcs_config(). It should check the guest's cpuid not this
> > unchanged value when get/set msr.
> >
> > Besides, vmx_compute_secondary_exec_control() adjusts
> > SECONDARY_EXEC_XSAVES bit based on guest cpuid's X86_FEATURE_XSAVE
> > and X86_FEATURE_XSAVES, it should use updated value to decide whether
> > set XSS_EXIT_BITMAP.
> >
> > Co-developed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxxxxxxxx>
> > Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxxxxxxxx>
> > Signed-off-by: Tao Xu <tao3.xu@xxxxxxxxx>
> > ---
> >   arch/x86/kvm/vmx/vmx.c | 8 +++++---
> >   1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index b93e36ddee5e..935cf72439a9 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -1721,7 +1721,8 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >                  return vmx_get_vmx_msr(&vmx->nested.msrs, msr_info->index,
> >                                         &msr_info->data);
> >          case MSR_IA32_XSS:
> > -               if (!vmx_xsaves_supported())
> > +               if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) ||
> > +                       !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
> >                          return 1;
> >                  msr_info->data = vcpu->arch.ia32_xss;
> >                  break;
> > @@ -1935,7 +1936,8 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >                          return 1;
> >                  return vmx_set_vmx_msr(vcpu, msr_index, data);
> >          case MSR_IA32_XSS:
> > -               if (!vmx_xsaves_supported())
> > +               if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) ||
> > +                       !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
> >                          return 1;
>
> Not complete true.
>
> >                  /*
> >                   * The only supported bit as of Skylake is bit 8, but
> > @@ -4094,7 +4096,7 @@ static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
> >
> >          set_cr4_guest_host_mask(vmx);
> >
> > -       if (vmx_xsaves_supported())
> > +       if (vmx->secondary_exec_control & SECONDARY_EXEC_XSAVES)
> >                  vmcs_write64(XSS_EXIT_BITMAP, VMX_XSS_EXIT_BITMAP);
>
> This is not true.
>
> SDM 24.6.20:
> On processors that support the 1-setting of the âenable
> XSAVES/XRSTORSâ VM-execution control, the VM-execution control fields
> include a 64-bit XSS-exiting bitmap.
>
> It depends on whether or not processors support the 1-setting instead
> of âenable XSAVES/XRSTORSâ is 1 in VM-exection control field. Anyway,

Yes, whether this field exist or not depends on whether processors 
support the 1-setting.

But if "enable XSAVES/XRSTORS" is clear to 0, XSS_EXIT_BITMAP doesn't 
work. I think in this case, there is no need to set this vmcs field?

> I will send a patch to fix the msr read/write for commit
> 203000993de5(kvm: vmx: add MSR logic for XSAVES), thanks for the
> report.
>
> Regards,
> Wanpeng Li