Re: [PATCH v2 09/21] KVM:x86: Load guest FPU state when accessing xsaves-managed MSRs

[Binbin Wu <binbin.wu@linux.intel.com>]

On 4/21/2023 9:46 PM, Yang Weijiang wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> Load the guest's FPU state if userspace is accessing MSRs whose values are
> managed by XSAVES so that the MSR helpers, e.g. kvm_{get,set}_xsave_msr(),
So far, kvm_{get,set}_xsave_msr() is not introduced yet.
IMO, it is a bit confusing to understand the whole picture without the 
following patches.
May be add some description or adjust the order?


> can simply do {RD,WR}MSR to access the guest's value.
>
> If new feature MSRs supported in XSS are passed through to the guest they
> are saved and restored by XSAVES/XRSTORS, i.e. in the guest's FPU state.
>
> Because 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@intel.com>
> Co-developed-by: Yang Weijiang <weijiang.yang@intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.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[] = {
> +	MSR_IA32_U_CET, MSR_IA32_PL3_SSP,
> +};
> +
> +static bool is_xsaves_msr(u32 index)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(xsave_msrs); i++) {
> +		if (index == xsave_msrs[i])
> +			return true;
> +	}
> +	return false;
> +}
> +
>   /*
>    * Read or write a bunch of msrs. All parameters are kernel addresses.
>    *
> @@ -4346,11 +4364,20 @@ static int __msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs *msrs,
>   		    int (*do_msr)(struct kvm_vcpu *vcpu,
>   				  unsigned index, u64 *data))
>   {
> +	bool fpu_loaded = false;
>   	int i;
>   
> -	for (i = 0; i < msrs->nmsrs; ++i)
> +	for (i = 0; i < msrs->nmsrs; ++i) {
> +		if (vcpu && !fpu_loaded && kvm_caps.supported_xss &&
> +		    is_xsaves_msr(entries[i].index)) {
> +			kvm_load_guest_fpu(vcpu);
> +			fpu_loaded = true;
> +		}
>   		if (do_msr(vcpu, entries[i].index, &entries[i].data))
>   			break;
> +	}
> +	if (fpu_loaded)
> +		kvm_put_guest_fpu(vcpu);
>   
>   	return i;
>   }