linux-kernel - [PATCH 08/19] KVM: x86: Load guest fpu state when accessing MSRs managed by XSAVES

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Date:   Thu, 16 Jun 2022 04:46:32 -0400
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     pbonzini@...hat.com, seanjc@...gle.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        rick.p.edgecombe@...el.com
Cc:     weijiang.yang@...el.com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: [PATCH 08/19] KVM: x86: Load guest fpu state when accessing MSRs managed by XSAVES

From: Sean Christopherson <sean.j.christopherson@...el.com>

If new feature MSRs are supported in XSS and passed through to the guest
they are saved and restored by XSAVES/XRSTORS, i.e. in the guest's FPU
state.

Load the guest's FPU state if userspace is accessing MSRs whose values are
managed by XSAVES so that the MSR helper, e.g. kvm_{get,set}_xsave_msr(),
can simply do {RD,WR}MSR to access the guest's value.

Because is also used for the KVM_GET_MSRS device ioctl(), explicitly
check that @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 | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 06fbd3daf393..506454e17afc 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -130,6 +130,9 @@ static int kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu);
 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 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)					     \
@@ -4138,6 +4141,11 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 }
 EXPORT_SYMBOL_GPL(kvm_get_msr_common);
 
+static bool is_xsaves_msr(u32 index)
+{
+	return index == MSR_IA32_U_CET || index == MSR_IA32_PL3_SSP;
+}
+
 /*
  * Read or write a bunch of msrs. All parameters are kernel addresses.
  *
@@ -4148,11 +4156,20 @@ 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;
 }
-- 
2.27.0