It turns out that our big real mode implementation was based on a miconception:
I believed that loading a segment register in real mode causes the limit to be
reset to 0xffff (thus undoing the effect), while in reality, the limit is preserved
across segment changes. This bug was hidden by another bug: we didn't do limit checks
in real mode. So even though segment loads corrupted the limit, the guest still
functioned because that limit isn't checked.
This patchset fixes both bugs, and introduces an optimization: we can now use
vm86 mode to virtualize big real mode (under usually-met conditions). This
both speeds up big real mode, and makes it more robust, since fewer instructions
need to be emulated.
I think this patchset means we can remove emulate_invalid_guest_state=0, since it
offers no advantages now.
Avi Kivity (13):
KVM: VMX: Separate saving pre-realmode state from setting segments
KVM: VMX: Fix incorrect lookup of segment S flag in
fix_pmode_dataseg()
KVM: VMX: Use kvm_segment to save protected-mode segments when
entering realmode
KVM: VMX: Retain limit and attributes when entering protected mode
KVM: VMX: Allow real mode emulation using vm86 with dpl=0
KVM: VMX: Allow vm86 virtualization of big real mode
KVM: x86 emulator: Leave segment limit and attributs alone in real
mode
KVM: x86 emulator: Check segment limits in real mode too
KVM: x86 emulator: Fix #GP error code during linearization
KVM: VMX: Return real real-mode segment data even if
emulate_invalid_guest_state=1
KVM: VMX: Preserve segment limit and access rights in real mode
KVM: VMX: Save all segment data in real mode
KVM: VMX: Ignore segment G and D bits when considering whether we can
virtualize
arch/x86/kvm/emulate.c | 19 ++++------
arch/x86/kvm/vmx.c | 100 ++++++++++++++++++-------------------------------
2 files changed, 44 insertions(+), 75 deletions(-)
--
1.7.11.3
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