From a1020a25e69755a8a1a37735d674b91d6f02939f Mon Sep 17 00:00:00 2001 From: "Dr. David Alan Gilbert" Date: Wed, 24 Aug 2022 03:30:56 +0000 Subject: [PATCH] KVM: x86: Always enable legacy FP/SSE in allowed user XFEATURES Allow FP and SSE state to be saved and restored via KVM_{G,SET}_XSAVE on XSAVE-capable hosts even if their bits are not exposed to the guest via XCR0. Failing to allow FP+SSE first showed up as a QEMU live migration failure, where migrating a VM from a pre-XSAVE host, e.g. Nehalem, to an XSAVE host failed due to KVM rejecting KVM_SET_XSAVE. However, the bug also causes problems even when migrating between XSAVE-capable hosts as KVM_GET_SAVE won't set any bits in user_xfeatures if XSAVE isn't exposed to the guest, i.e. KVM will fail to actually migrate FP+SSE. Because KVM_{G,S}ET_XSAVE are designed to allowing migrating between hosts with and without XSAVE, KVM_GET_XSAVE on a non-XSAVE (by way of fpu_copy_guest_fpstate_to_uabi()) always sets the FP+SSE bits in the header so that KVM_SET_XSAVE will work even if the new host supports XSAVE. Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0") bz: https://bugzilla.redhat.com/show_bug.cgi?id=2079311 Cc: stable@vger.kernel.org Cc: Leonardo Bras Signed-off-by: Dr. David Alan Gilbert [sean: add comment, massage changelog] Signed-off-by: Sean Christopherson Message-Id: <20220824033057.3576315-3-seanjc@google.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/cpuid.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index 2e0f27a..4c1c2c0 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -329,7 +329,13 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) vcpu->arch.guest_supported_xcr0 = cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent); - vcpu->arch.guest_fpu.fpstate->user_xfeatures = vcpu->arch.guest_supported_xcr0; + /* + * FP+SSE can always be saved/restored via KVM_{G,S}ET_XSAVE, even if + * XSAVE/XCRO are not exposed to the guest, and even if XSAVE isn't + * supported by the host. + */ + vcpu->arch.guest_fpu.fpstate->user_xfeatures = vcpu->arch.guest_supported_xcr0 | + XFEATURE_MASK_FPSSE; kvm_update_pv_runtime(vcpu); -- 2.7.4