KVM: Synthesize G bit for all segments.

We have noticed that qemu-kvm hangs early in the BIOS when runnning nested
under some versions of VMware ESXi.

The problem we believe is because KVM assumes that the platform preserves
the 'G' but for any segment register. The SVM specification itemizes the
segment attribute bits that are observed by the CPU, but the (G)ranularity bit
is not one of the bits itemized, for any segment. Though current AMD CPUs keep
track of the (G)ranularity bit for all segment registers other than CS, the
specification does not require it. VMware's virtual CPU may not track the
(G)ranularity bit for any segment register.

Since kvm already synthesizes the (G)ranularity bit for the CS segment. It
should do so for all segments. The patch below does that, and helps get rid of
the hangs. Patch applies on top of Linus' tree.

Signed-off-by: Jim Mattson <jmattson@vmware.com>
Signed-off-by: Alok N Kataria <akataria@vmware.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 10 insertions, 9 deletions

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 85d4458a0b35..4925a94a970a 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1415,7 +1415,16 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
 	var->avl = (s->attrib >> SVM_SELECTOR_AVL_SHIFT) & 1;
 	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;
+
+	/*
+	 * AMD CPUs circa 2014 track the G bit for all segments except CS.
+	 * However, the SVM spec states that the G bit is not observed by the
+	 * CPU, and some VMware virtual CPUs drop the G bit for all segments.
+	 * So let's synthesize a legal G bit for all segments, this helps
+	 * running KVM nested. It also helps cross-vendor migration, because
+	 * Intel's vmentry has a check on the 'G' bit.
+	 */
+	var->g = s->limit > 0xfffff;
 
 	/*
 	 * AMD's VMCB does not have an explicit unusable field, so emulate it
@@ -1424,14 +1433,6 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
 	var->unusable = !var->present || (var->type == 0);
 
 	switch (seg) {
-	case VCPU_SREG_CS:
-		/*
-		 * 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.
-		 */
-		var->g = s->limit > 0xfffff;
-		break;
 	case VCPU_SREG_TR:
 		/*
 		 * Work around a bug where the busy flag in the tr selector