linux-kernel - [PATCH] KVM: SVM: ignore type when setting segment registers

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Date:   Tue, 30 May 2017 15:24:45 +0200
From:   Gioh Kim <gi-oh.kim@...fitbricks.com>
To:     joro@...tes.org, pbonzini@...hat.com, rkrcmar@...hat.com
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Gioh Kim <gi-oh.kim@...fitbricks.com>
Subject: [PATCH] KVM: SVM: ignore type when setting segment registers

Commit 19bca6ab75d8 ("KVM: SVM: Fix cross vendor migration issue with
unusable bit") added checking type when setting unusable.
So unusable can be set if present is 0 OR type is 0.
According to the AMD processor manual, long mode ignores the type value
in segment descriptor. And type can be 0 if it is read-only data segment.
Therefore type value is not related to unusable flag.

This patch is based on linux-next v4.12.0-rc3.

Signed-off-by: Gioh Kim <gi-oh.kim@...fitbricks.com>
---
 arch/x86/kvm/svm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 183ddb2..a654372 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1807,7 +1807,7 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
 	 * 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);
+	var->unusable = !var->present;

 	switch (seg) {
 	case VCPU_SREG_TR:
-- 
2.5.0