linux-kernel - Re: [PATCH 42/43] KVM: VMX: Drop VMWRITEs to zero fields at vCPU RESET

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Date:   Mon, 24 May 2021 15:48:04 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 42/43] KVM: VMX: Drop VMWRITEs to zero fields at vCPU RESET

On Mon, May 24, 2021 at 3:28 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Mon, May 24, 2021, Paolo Bonzini wrote:
> > On 24/04/21 02:46, Sean Christopherson wrote:
> > > Don't waste time writing zeros via VMWRITE during vCPU RESET, the VMCS
> > > is zero allocated.
> >
> > Is this guaranteed to be valid, or could the VMCS in principle use some
> > weird encoding? (Like it does for the access rights, even though this does
> > not matter for this patch).
>
> Phooey.  In principle, the CPU can do whatever it wants, e.g. the SDM states that
> software should never write to the data portion of the VMCS under any circumstance.
>
> In practice, I would be flabbergasted if Intel ever ships a CPU that doesn't play
> nice with zero initiazing the VMCS via software writes.  I'd bet dollars to
> donuts that KVM isn't the only software that relies on that behavior.

It's not just Intel. It's any manufacturer of physical or virtual CPUs
that implement VT-x. Non-architected behavior isn't guaranteed.

> That said, I'm not against switching to VMWRITE for everything, but regardless
> of which route we choose, we should commit to one or the other.  I.e. double down
> on memset() and bet that Intel won't break KVM, or replace the memset() in
> alloc_vmcs_cpu() with a sequence that writes all known (possible?) fields.  The
> current approach of zeroing the memory in software but initializing _some_ fields
> is the worst option, e.g. I highly doubt vmcs01 and vmcs02 do VMWRITE(..., 0) on
> the same fields.

The memset should probably be dropped, unless it is there to prevent
information leakage. However, it is not necessary to VMWRITE all known
(or possible) fields--just those that aren't guarded by an enable bit.