linux-kernel - Re: [PATCH] kvm: x86: nVMX: maintain internal copy of current VMCS

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Date:	Fri, 15 Jul 2016 10:54:13 -0700
From:	David Matlack <dmatlack@...gle.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	kvm list <kvm@...r.kernel.org>, Jim Mattson <jmattson@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kvm: x86: nVMX: maintain internal copy of current VMCS

On Thu, Jul 14, 2016 at 1:33 AM, Paolo Bonzini <pbonzini@...hat.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@...gle.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