linux-kernel - Re: [PATCH] KVM: vmx: Fix the broken usage of vmx_xsaves

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Date:   Thu, 20 Jun 2019 14:40:15 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Tao Xu <tao3.xu@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, xiaoyao.li@...ux.intel.com
Subject: Re: [PATCH] KVM: vmx: Fix the broken usage of vmx_xsaves_supported

Hi,
On Thu, 20 Jun 2019 at 13:06, Tao Xu <tao3.xu@...el.com> 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@...ux.intel.com>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...ux.intel.com>
> Signed-off-by: Tao Xu <tao3.xu@...el.com>
> ---
>  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 “enable
XSAVES/XRSTORS” 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 “enable XSAVES/XRSTORS” is 1 in VM-exection control field. Anyway,
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