linux-kernel - Re: [PATCH v7 5/7] kvm: x86: Add CET CR4 bit and XSS support

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Date:   Wed, 2 Oct 2019 12:05:23 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v7 5/7] kvm: x86: Add CET CR4 bit and XSS support

On Thu, Sep 26, 2019 at 7:17 PM Yang Weijiang <weijiang.yang@...el.com> wrote:
>
> CR4.CET(bit 23) is master enable bit for CET feature.
> Previously, KVM did not support setting any bits in XSS
> so it's hardcoded to check and inject a #GP if Guest
> attempted to write a non-zero value to XSS, now it supports
> CET related bits setting.
>
> Co-developed-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  4 +++-
>  arch/x86/kvm/cpuid.c            | 11 +++++++++--
>  arch/x86/kvm/vmx/vmx.c          |  6 +-----
>  3 files changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index d018df8c5f32..8f97269d6d9f 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -90,7 +90,8 @@
>                           | X86_CR4_PGE | X86_CR4_PCE | X86_CR4_OSFXSR | X86_CR4_PCIDE \
>                           | X86_CR4_OSXSAVE | X86_CR4_SMEP | X86_CR4_FSGSBASE \
>                           | X86_CR4_OSXMMEXCPT | X86_CR4_LA57 | X86_CR4_VMXE \
> -                         | X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP))
> +                         | X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP \
> +                         | X86_CR4_CET))
>
>  #define CR8_RESERVED_BITS (~(unsigned long)X86_CR8_TPR)
>
> @@ -623,6 +624,7 @@ struct kvm_vcpu_arch {
>
>         u64 xcr0;
>         u64 guest_supported_xcr0;
> +       u64 guest_supported_xss;
>         u32 guest_xstate_size;
>
>         struct kvm_pio_request pio;
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0a47b9e565be..dd3ddc6daa58 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -120,8 +120,15 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>         }
>
>         best = kvm_find_cpuid_entry(vcpu, 0xD, 1);
> -       if (best && (best->eax & (F(XSAVES) | F(XSAVEC))))
> -               best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> +       if (best && (best->eax & (F(XSAVES) | F(XSAVEC)))) {

Is XSAVEC alone sufficient? Don't we explicitly need XSAVES to
save/restore the extended state components enumerated by IA32_XSS?

> +               u64 kvm_xss = kvm_supported_xss();
> +
> +               best->ebx =
> +                       xstate_required_size(vcpu->arch.xcr0 | kvm_xss, true);

Shouldn't this size be based on the *current* IA32_XSS value, rather
than the supported IA32_XSS bits? (i.e.
s/kvm_xss/vcpu->arch.ia32_xss/)

> +               vcpu->arch.guest_supported_xss = best->ecx & kvm_xss;

Shouldn't unsupported bits in best->ecx be masked off, so that the
guest CPUID doesn't mis-report the capabilities of the vCPU?

> +       } else {
> +               vcpu->arch.guest_supported_xss = 0;
> +       }
>
>         /*
>          * The existing code assumes virtual address is 48-bit or 57-bit in the
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ba1a83d11e69..44913e4ab558 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1973,11 +1973,7 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>                      !(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)
> +               if (data & ~vcpu->arch.guest_supported_xss)
>                         return 1;
>                 vcpu->arch.ia32_xss = data;
>                 if (vcpu->arch.ia32_xss != host_xss)
> --
> 2.17.2
>