[PATCH 37/47] KVM: When switching to a vm8086 task, load segments as 16-bit

From: Avi Kivity
Date: Wed Aug 26 2009 - 06:37:47 EST

From: Anthony Liguori <aliguori@xxxxxxxxxx>

According to 16.2.5 in the SDM, eflags.vm in the tss is consulted before loading
and new segments. If eflags.vm == 1, then the segments are treated as 16-bit
segments. The LDTR and TR are not normally available in vm86 mode so if they
happen to somehow get loaded, they need to be treated as 32-bit segments.

This fixes an invalid vmentry failure in a custom OS that was happening after
a task switch into vm8086 mode. Since the segments were being mistakenly
treated as 32-bit, we loaded garbage state.

Signed-off-by: Anthony Liguori <aliguori@xxxxxxxxxx>
Signed-off-by: Avi Kivity <avi@xxxxxxxxxx>
arch/x86/kvm/x86.c | 9 ++++++++-
1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 31bf984..1aa7e6d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4101,12 +4101,19 @@ static int kvm_load_realmode_segment(struct kvm_vcpu *vcpu, u16 selector, int se
return 0;

+static int is_vm86_segment(struct kvm_vcpu *vcpu, int seg)
+ return (seg != VCPU_SREG_LDTR) &&
+ (seg != VCPU_SREG_TR) &&
+ (kvm_x86_ops->get_rflags(vcpu) & X86_EFLAGS_VM);
int kvm_load_segment_descriptor(struct kvm_vcpu *vcpu, u16 selector,
int type_bits, int seg)
struct kvm_segment kvm_seg;

- if (!(vcpu->arch.cr0 & X86_CR0_PE))
+ if (is_vm86_segment(vcpu, seg) || !(vcpu->arch.cr0 & X86_CR0_PE))
return kvm_load_realmode_segment(vcpu, selector, seg);
if (load_segment_descriptor_to_kvm_desct(vcpu, selector, &kvm_seg))
return 1;

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/