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Date:   Fri, 8 Sep 2023 10:18:48 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Yan Zhao <yan.y.zhao@intel.com>, kvm@vger.kernel.org,
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        Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 0/2] KVM: x86/mmu: .change_pte() optimization in TDP MMU
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On Wed, Sep 06, 2023 at 05:18:51PM +0100, Robin Murphy wrote:
> 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?

Well, the fix for bounce buffering is to trust the device, and there is
a lot of work going into device authentication and attesttion right now
so that will happen.

On the swiotlb side a new version of the dma_sync_*_device APIs that
specifies the mapping len and the data length transfer would avoid
some of the overhead here.  We've decided that it is a good idea last
time, but so far no one has volunteers to implement it.