linux-kernel - Re: [PATCH 0/2] KVM: x86/mmu: .change

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Message-ID: <ZPis61o4lkjr0mMU@google.com>
Date:   Wed, 6 Sep 2023 09:46:35 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Yan Zhao <yan.y.zhao@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/2] KVM: x86/mmu: .change_pte() optimization in TDP MMU

On Wed, Sep 06, 2023, Robin Murphy wrote:
> On 2023-09-06 15:44, Sean Christopherson wrote:
> > On Wed, Sep 06, 2023, Robin Murphy wrote:
> > > Even non-virtualised, SWIOTLB is pretty horrible for I/O performance by its
> > > very nature - avoiding it if at all possible should always be preferred.
> > 
> > Yeah.  The main reason I didn't just sweep this under the rug is the confidential
> > VM use case, where SWIOTLB is used to bounce data from guest private memory into
> > shread buffers.  There's also a good argument that anyone that cares about I/O
> > performance in confidential VMs should put in the effort to enlighten their device
> > drivers to use shared memory directly, but practically speaking that's easier said
> > than done.
> 
> Indeed a bunch of work has gone into SWIOTLB recently trying to make it a
> bit more efficient for such cases where it can't be avoided, so it is
> definitely still interesting to learn about impacts at other levels like
> this. Maybe there's a bit of a get-out for confidential VMs though, since
> presumably there's not much point COW-ing encrypted private memory, so
> perhaps KVM might end up wanting to optimise that out and thus happen to end
> up less sensitive to unavoidable SWIOTLB behaviour anyway?

CoW should be a non-issue for confidential VMs, at least on x86.  SEV and SEV-ES
are effectively forced to pin memory as writable before it can be mapped into the
guest.  TDX and SNP and will have a different implementation, but similar behavior.

Confidential VMs would benefit purely by either eliminating or reducing the cost
of "initializing" memory, i.e. by eliminating the memcpy() or replacing it with a
memset().  I most definitely don't care enough about confidential VM I/O performance
to try and micro-optimize that behavior, their existence was simply what made me
look more closely instead of just telling Yan to stop using IDE :-)