Messages in this thread | | | From | David Matlack <> | Date | Fri, 15 Jul 2016 10:54:13 -0700 | Subject | Re: [PATCH] kvm: x86: nVMX: maintain internal copy of current VMCS |
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On Thu, Jul 14, 2016 at 1:33 AM, Paolo Bonzini <pbonzini@redhat.com> wrote: > > > On 14/07/2016 02:16, David Matlack wrote: >> KVM maintains L1's current VMCS in guest memory, at the guest physical >> page identified by the argument to VMPTRLD. This makes hairy >> time-of-check to time-of-use bugs possible,as VCPUs can be writing >> the the VMCS page in memory while KVM is emulating VMLAUNCH and >> VMRESUME. >> >> The spec documents that writing to the VMCS page while it is loaded is >> "undefined". Therefore it is reasonable to load the entire VMCS into >> an internal cache during VMPTRLD and ignore writes to the VMCS page >> -- the guest should be using VMREAD and VMWRITE to access the current >> VMCS. >> >> To adhere to the spec, KVM should flush the current VMCS during VMPTRLD, >> and the target VMCS during VMCLEAR (as given by the operand to VMCLEAR). >> Since this implementation of VMCS caching only maintains the the current >> VMCS, VMCLEAR will only do a flush if the operand to VMCLEAR is the >> current VMCS pointer. >> >> KVM will also flush during VMXOFF, which is not mandated by the spec, >> but also not in conflict with the spec. >> >> Signed-off-by: David Matlack <dmatlack@google.com> > > This is a good change. There is another change that is possible on top: > with this change you don't need current_vmcs12/current_vmcs12_page at > all, I think. You can just use current_vmptr and kvm_read/write_guest > to write back the VMCS12, possibly the cached variants.
Good catch, I agree they can be removed.
> > Of course this would just be a small simplification, so I'm applying the > patch as is to kvm/next.
SGTM. Thanks for the review.
> > Thanks, > > Paolo
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