linux-kernel - Re: [PATCH v3 09/21] KVM:x86: Load guest FPU state when accessing xsaves-managed MSRs

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Message-ID: <ZIujqYFd3xR2oPwi@google.com>
Date:   Thu, 15 Jun 2023 16:50:01 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        rppt@...nel.org, binbin.wu@...ux.intel.com,
        rick.p.edgecombe@...el.com, john.allen@....com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v3 09/21] KVM:x86: Load guest FPU state when accessing
 xsaves-managed MSRs

On Thu, May 11, 2023, Yang Weijiang wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> Load the guest's FPU state if userspace is accessing MSRs whose values are
> managed by XSAVES. Two MSR access helpers, i.e., kvm_{get,set}_xsave_msr(),
> are introduced by a later patch to facilitate access to this kind of MSRs.
> 
> If new feature MSRs supported in XSS are passed through to the guest they
> are saved and restored by {XSAVES|XRSTORS} to/from guest's FPU state at
> vm-entry/exit.
> 
> Because the modified code is also used for the KVM_GET_MSRS device ioctl(),
> explicitly check @vcpu is non-null before attempting to load guest state.
> The XSS supporting MSRs cannot be retrieved via the device ioctl() without
> loading guest FPU state (which doesn't exist).
> 
> Note that guest_cpuid_has() is not queried as host userspace is allowed
> to access MSRs that have not been exposed to the guest, e.g. it might do
> KVM_SET_MSRS prior to KVM_SET_CPUID2.
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Co-developed-by: Yang Weijiang <weijiang.yang@...el.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>  arch/x86/kvm/x86.c | 29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d2975ca96ac5..7788646bbf1f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -130,6 +130,9 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
>  static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
>  
>  static DEFINE_MUTEX(vendor_module_lock);
> +static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu);
> +static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu);
> +
>  struct kvm_x86_ops kvm_x86_ops __read_mostly;
>  
>  #define KVM_X86_OP(func)					     \
> @@ -4336,6 +4339,21 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  }
>  EXPORT_SYMBOL_GPL(kvm_get_msr_common);
>  
> +static const u32 xsave_msrs[] = {

Can you change this to "xstate_msrs"?


> +	MSR_IA32_U_CET, MSR_IA32_PL3_SSP,
> +};
> +
> +static bool is_xsaves_msr(u32 index)

And then is_xstate_msr().  The intent to is check if an MSR is managed as part of
the xstate, not if the MSR is somehow related to XSAVE itself.