[PATCH 1/2] Fix cross vendor migration issue with unusable bit

[PATCH 1/2] Fix cross vendor migration issue with unusable bit

[Andre Przywara]

AMDs VMCB does not have an explicit unusable segment descriptor field,
so we emulate it by using "not present". This has to be setup before
the fixups, because this field is used there.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
---
 arch/x86/kvm/svm.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

Avi, pleasy push this patch also to 2.6.30, since it fixes a bug introduced
in 2.6.30-rc1.

Thanks,
Andre.

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 1821c20..1580464 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -796,6 +796,11 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
 	var->db = (s->attrib >> SVM_SELECTOR_DB_SHIFT) & 1;
 	var->g = (s->attrib >> SVM_SELECTOR_G_SHIFT) & 1;
 
+	/* AMD's VMCB does not have an explicit unusable field, so emulate it
+	 * for cross vendor migration purposes by "not present"
+	 */
+	var->unusable = !var->present || (var->type == 0);
+	
 	switch (seg) {
 	case VCPU_SREG_CS:
 		/*
@@ -827,8 +832,6 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
 			var->type |= 0x1;
 		break;
 	}
-
-	var->unusable = !var->present;
 }
 
 static int svm_get_cpl(struct kvm_vcpu *vcpu)
-- 
1.6.1.3

Re: [PATCH 1/2] Fix cross vendor migration issue with unusable bit

[Avi Kivity]

Andre Przywara wrote:
> AMDs VMCB does not have an explicit unusable segment descriptor field,
> so we emulate it by using "not present". This has to be setup before
> the fixups, because this field is used there.
>
>   

Applied both, thanks.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.