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Date:   Fri, 23 Apr 2021 16:14:32 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, xieyongji@bytedance.com,
        stefanha@redhat.com, file@sect.tu-berlin.de, ashish.kalra@amd.com,
        martin.radev@aisec.fraunhofer.de, konrad.wilk@oracle.com,
        kvm@vger.kernel.org
Subject: Re: [RFC PATCH 0/7] Untrusted device support for virtio
Message-ID: <20210423161114-mutt-send-email-mst@kernel.org>
References: <20210421032117.5177-1-jasowang@redhat.com>
 <20210422063128.GB4176641@infradead.org>
 <0c61dcbb-ac5b-9815-a4a1-5f93ae640011@redhat.com>
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On Thu, Apr 22, 2021 at 04:19:16PM +0800, Jason Wang wrote:
> 
> 在 2021/4/22 下午2:31, Christoph Hellwig 写道:
> > On Wed, Apr 21, 2021 at 11:21:10AM +0800, Jason Wang wrote:
> > > The behaivor for non DMA API is kept for minimizing the performance
> > > impact.
> > NAK.  Everyone should be using the DMA API in a modern world.  So
> > treating the DMA API path worse than the broken legacy path does not
> > make any sense whatsoever.
> 
> 
> I think the goal is not treat DMA API path worse than legacy. The issue is
> that the management layer should guarantee that ACCESS_PLATFORM is set so
> DMA API is guaranteed to be used by the driver. So I'm not sure how much
> value we can gain from trying to 'fix' the legacy path. But I can change the
> behavior of legacy path to match DMA API path.
> 
> Thanks

I think before we maintain different paths with/without ACCESS_PLATFORM
it's worth checking whether it's even a net gain. Avoiding sharing
by storing data in private memory can actually turn out to be
a net gain even without DMA API.

It is worth checking what is the performance effect of this patch.


-- 
MST