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

Maxim Levitsky
2021-04-01T18:08:57Z

On Thu, 2021-04-01 at 14:16 +0300, Maxim Levitsky wrote:
> 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.

Please ignore this patch series, I didn't update the patch version.

Best regards,
Maxim Levitsky

>
> 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(-)
>
> --
> 2.26.2
>