linux-kernel - [PATCH 2/3] KVM: x86: Always enable legacy FP/SSE in allowed user XFEATURES

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Message-Id: <20220824033057.3576315-3-seanjc@google.com>
Date:   Wed, 24 Aug 2022 03:30:56 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Leonardo Bras <leobras@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: [PATCH 2/3] KVM: x86: Always enable legacy FP/SSE in allowed user XFEATURES

From: Dr. David Alan Gilbert <dgilbert@...hat.com>

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@...r.kernel.org
Cc: Leonardo Bras <leobras@...hat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@...hat.com>
[sean: add comment, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 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 2e0f27ad736a..4c1c2c06e96b 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);

-- 
2.37.1.595.g718a3a8f04-goog