Fix Linux guests migration from AMD to Intel hosts

Fix Linux guests migration from AMD to Intel hosts

[Amit Shah]

These two patches export some bits from SVM's VMCB to the state
that enable migration from AMD hosts to Intel hosts.

x86 Linux-32 bit and 64-bit guests successfully migrate after
applying these patches. Note: Intel->AMD migration of guests
already works.

[PATCH 1/2] KVM: SVM: Set the 'g' bit of the cs selector for cross-vendor migration

[Amit Shah]

The hardware does not set the 'g' bit of the cs selector and this breaks
migration from amd hosts to intel hosts. Set this bit if the segment
limit is beyond 1 MB.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
---
 arch/x86/kvm/svm.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 05efc4e..665008d 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -772,6 +772,15 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
 	var->l = (s->attrib >> SVM_SELECTOR_L_SHIFT) & 1;
 	var->db = (s->attrib >> SVM_SELECTOR_DB_SHIFT) & 1;
 	var->g = (s->attrib >> SVM_SELECTOR_G_SHIFT) & 1;
+
+	/*
+	 * SVM always stores 0 for the 'G' bit in the CS selector in
+	 * the VMCB on a VMEXIT. This hurts cross-vendor migration:
+	 * Intel's VMENTRY has a check on the 'G' bit.
+	 */
+	if (seg == VCPU_SREG_CS)
+		var->g = s->limit > 0xfffff;
+
 	var->unusable = !var->present;
 }
 
-- 
1.5.4.3

[PATCH 2/2] KVM: SVM: Set the 'busy' flag of the TR selector

[Amit Shah]

The busy flag of the TR selector is not set by the hardware. This breaks
migration from amd hosts to intel hosts.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
---
 arch/x86/kvm/svm.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 665008d..743aebd 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -781,6 +781,13 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
 	if (seg == VCPU_SREG_CS)
 		var->g = s->limit > 0xfffff;
 
+	/*
+	 * Work around a bug where the busy flag in the tr selector
+	 * isn't exposed
+	 */
+	if (seg == VCPU_SREG_TR)
+		var->type |= 0x2;
+
 	var->unusable = !var->present;
 }
 
-- 
1.5.4.3

Re: Fix Linux guests migration from AMD to Intel hosts

[Avi Kivity]

Amit Shah wrote:
> These two patches export some bits from SVM's VMCB to the state
> that enable migration from AMD hosts to Intel hosts.
>
> x86 Linux-32 bit and 64-bit guests successfully migrate after
> applying these patches. Note: Intel->AMD migration of guests
> already works.
>   

Applied, thanks.

-- 
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