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From:   "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>
To:     kvm@vger.kernel.org, seanjc@google.com, pbonzini@redhat.com,
        tglx@linutronix.de, leobras@redhat.com,
        linux-kernel@vger.kernel.org
Cc:     mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        x86@kernel.org
Subject: [PATCH] KVM: x86: Always enable legacy fp/sse
Date:   Tue, 16 Aug 2022 18:59:36 +0100
Message-Id: <20220816175936.23238-1-dgilbert@redhat.com>
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From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

A live migration under qemu is currently failing when the source
host is ~Nehalem era (pre-xsave) and the destination is much newer,
(configured with a guest CPU type of Nehalem).
QEMU always calls kvm_put_xsave, even on this combination because
KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE.

When QEMU calls kvm_put_xsave it's rejected by
   fpu_copy_uabi_to_guest_fpstate->
     copy_uabi_to_xstate->
       validate_user_xstate_header

when the validate checks the loaded xfeatures against
user_xfeatures, which it finds to be 0.

I think our initialisation of user_xfeatures is being
too strict here, and we should always allow the base FP/SSE.

Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0")
bz: https://bugzilla.redhat.com/show_bug.cgi?id=2079311

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 arch/x86/kvm/cpuid.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index de6d44e07e34..3b2319cecfd1 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -298,7 +298,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	guest_supported_xcr0 =
 		cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
 
-	vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0;
+	vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0 |
+		XFEATURE_MASK_FPSSE;
 
 	kvm_update_pv_runtime(vcpu);
 
-- 
2.37.2