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

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Date:	Thu, 14 Jul 2016 10:33:50 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	David Matlack <dmatlack@...gle.com>, kvm@...r.kernel.org
Cc:	jmattson@...gle.com, pfeiner@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: nVMX: maintain internal copy of current VMCS



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.

Of course this would just be a small simplification, so I'm applying the
patch as is to kvm/next.

Thanks,

Paolo