[RFC PATCH 08/13] KVM: planes: Enlarge every twin's fpstate for dynamic xfeatures

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Plane twins share CPUID, so a twin's guest can set the XCR0 bits for a
dynamic xfeature such as AMX's XTILEDATA with its own XSETBV. But
kvm_check_cpuid() calls fpu_enable_guest_xfd_features() only for the
vCPU that KVM_SET_CPUID was issued on, and a twin created afterwards
starts with the default fpstate. A twin that then runs AMX code loses
its tile data on every fpstate save, because the fpstate it saves into
has no room for it. On a host that exposes AMX to the guest, any twin
using AMX hits this.

Whether the XRSTORS on the next entry can also leave a previous
context's tile registers in place for that twin to read is an open
question; it has not been verified.

Enlarge the fpstate of every twin in kvm_check_cpuid(), and give a twin
created after KVM_SET_CPUID the dynamic xfeatures of its plane-0
sibling's fpstate.

Only a twin that has not run may have its fpstate enlarged:
fpstate_realloc() hands a guest fpstate that is not loaded a fresh
image and copies nothing, so a twin that has run would lose its XSAVE
state. kvm_set_cpuid() already accepts only identical CPUID once the
vCPU has run, but it checked only the vCPU the ioctl was issued on.
With plane 0 held while a higher plane runs, KVM_SET_CPUID2 on plane 0
would pass that check and resize the plane that ran. Since the twins
share CPUID, take the identical-CPUID path once any twin has run.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 arch/x86/kvm/cpuid.c | 21 +++++++++++++++++++--
 arch/x86/kvm/x86.c   | 17 +++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index c5fa0551ddd5..971ebd781c6e 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -141,7 +141,9 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_find_cpuid_entry2);
 static int kvm_check_cpuid(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *best;
+	struct kvm_vcpu *v;
 	u64 xfeatures;
+	int i, r;
 
 	/*
 	 * The existing code assumes virtual address is 48-bit or 57-bit in the
@@ -168,7 +170,18 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu)
 	if (!xfeatures)
 		return 0;
 
-	return fpu_enable_guest_xfd_features(&vcpu->arch.guest_fpu, xfeatures);
+	/*
+	 * The planes share CPUID, so any twin's guest can enable these
+	 * xfeatures in its XCR0: enlarge every twin's fpstate, not just
+	 * this vCPU's.
+	 */
+	vcpu_for_each_plane(vcpu->common, i, v) {
+		r = fpu_enable_guest_xfd_features(&v->arch.guest_fpu, xfeatures);
+		if (r)
+			return r;
+	}
+
+	return 0;
 }
 
 static u32 kvm_apply_cpuid_pv_features_quirk(struct kvm_vcpu *vcpu);
@@ -545,8 +558,12 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 	 * VMMs (e.g. QEMU) reuse vCPU fds for CPU hotplug/unplug and do
 	 * KVM_SET_CPUID{,2} again. To support this legacy behavior, check
 	 * whether the supplied CPUID data is equal to what's already set.
+	 * Plane twins share CPUID, so this applies once any twin has run.
 	 */
-	if (!kvm_can_set_cpuid_and_feature_msrs(vcpu)) {
+	vcpu_for_each_plane(common, i, v) {
+		if (kvm_can_set_cpuid_and_feature_msrs(v))
+			continue;
+
 		r = kvm_cpuid_check_equal(vcpu, e2, nent);
 		if (r)
 			goto err;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6d3d93e85a65..af28e73e4b9b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9771,6 +9771,23 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 	vcpu->arch.hv_root_tdp = INVALID_PAGE;
 #endif
 
+	/*
+	 * A plane twin created after KVM_SET_CPUID shares CPUID that may
+	 * already allow dynamic xfeatures, which KVM_SET_CPUID enabled only
+	 * in the twins that existed at the time.  Give it the dynamic
+	 * xfeatures of its plane-0 sibling's fpstate.
+	 */
+	if (vcpu->plane_level && vcpu->common->vcpus[0]) {
+		u64 dyn = vcpu->common->vcpus[0]->arch.guest_fpu.fpstate->xfeatures &
+			  XFEATURE_MASK_USER_DYNAMIC;
+
+		if (dyn) {
+			r = fpu_enable_guest_xfd_features(&vcpu->arch.guest_fpu, dyn);
+			if (r)
+				goto free_guest_fpu;
+		}
+	}
+
 	/*
 	 * A plane twin created after KVM_SET_CPUID shares CPUID that may
 	 * already advertise Hyper-V.  Allocate the Hyper-V context now so
-- 
2.50.1