[PATCH 0/2] KVM: x86: nSVM: fixes for SYSENTER emulation

Maxim Levitsky
2021-04-01T17:56:50Z

This is a result of a deep rabbit hole dive in regard to why
currently the nested migration of 32 bit guests
is totally broken on AMD.

It turns out that due to slight differences between the original AMD64
implementation and the Intel's remake, SYSENTER instruction behaves a
bit differently on Intel, and to support migration from Intel to AMD we
try to emulate those differences away.

Sadly that collides with virtual vmload/vmsave feature that is used in nesting.
The problem was that when it is enabled,
on migration (and otherwise when userspace reads MSR_IA32_SYSENTER_{EIP|ESP},
wrong value were returned, which leads to #DF in the
nested guest when the wrong value is loaded back.

The patch I prepared carefully fixes this, by mostly disabling that
SYSCALL emulation when we don't spoof the Intel's vendor ID, and if we do,
and yet somehow SVM is enabled (this is a very rare edge case), then
virtual vmload/save is force disabled.

V2: incorporated review feedback from Paulo.

Best regards,
Maxim Levitsky

Maxim Levitsky (2):
KVM: x86: add guest_cpuid_is_intel
KVM: nSVM: improve SYSENTER emulation on AMD

arch/x86/kvm/cpuid.h | 8 ++++
arch/x86/kvm/svm/svm.c | 99 +++++++++++++++++++++++++++---------------
arch/x86/kvm/svm/svm.h | 6 +--
3 files changed, 76 insertions(+), 37 deletions(-)

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2.26.2