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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 <leobras@redhat.com> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> [sean: add comment, massage changelog] Signed-off-by: Sean Christopherson <seanjc@google.com> Message-Id: <20220824033057.3576315-3-seanjc@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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@ -329,7 +329,13 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
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vcpu->arch.guest_supported_xcr0 =
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cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
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vcpu->arch.guest_fpu.fpstate->user_xfeatures = vcpu->arch.guest_supported_xcr0;
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/*
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* FP+SSE can always be saved/restored via KVM_{G,S}ET_XSAVE, even if
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* XSAVE/XCRO are not exposed to the guest, and even if XSAVE isn't
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* supported by the host.
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*/
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vcpu->arch.guest_fpu.fpstate->user_xfeatures = vcpu->arch.guest_supported_xcr0 |
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XFEATURE_MASK_FPSSE;
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kvm_update_pv_runtime(vcpu);
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