Re: [PATCH 0/2] KVM: nVMX: vmcs.SYSENTER optimization and "fix"

[Jim Mattson]

On Tue, Apr 28, 2020 at 4:10 PM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> Patch 1 is a "fix" for handling SYSENTER_EIP/ESP in L2 on a 32-bit vCPU.
> The primary motivation is to provide consistent behavior after patch 2.
>
> Patch 2 is essentially a re-submission of a nested VMX optimization to
> avoid redundant VMREADs to the SYSENTER fields in the nested VM-Exit path.
>
> After patch 2 and without patch 1, KVM would end up with weird behavior
> where L1 and L2 would only see 32-bit values for their own SYSENTER_E*P
> MSRs, but L1 could see a 64-bit value for L2's MSRs.
>
> Sean Christopherson (2):
>   KVM: nVMX: Truncate writes to vmcs.SYSENTER_EIP/ESP for 32-bit vCPU
>   KVM: nVMX: Drop superfluous VMREAD of vmcs02.GUEST_SYSENTER_*
>
>  arch/x86/kvm/vmx/nested.c |  4 ----
>  arch/x86/kvm/vmx/vmx.c    | 18 ++++++++++++++++--
>  2 files changed, 16 insertions(+), 6 deletions(-)

It seems like this could be fixed more generally by truncating
natural-width fields on 32-bit vCPUs in handle_vmwrite(). However,
that also would imply that we can't shadow any natural-width fields on
a 32-bit vCPU.