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Date:   Mon, 24 Feb 2020 18:16:57 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Halil Pasic <pasic@linux.ibm.com>, Christoph Hellwig <hch@lst.de>,
        Jason Wang <jasowang@redhat.com>,
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        "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
        Michael Mueller <mimu@linux.ibm.com>
Subject: Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is
 protected
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References: <20200220160606.53156-1-pasic@linux.ibm.com> <20200220160606.53156-3-pasic@linux.ibm.com> <20200220161309.GB12709@lst.de> <20200221153340.4cdcde81.pasic@linux.ibm.com> <20200222140408-mutt-send-email-mst@kernel.org>
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On Sat, Feb 22, 2020 at 02:07:58PM -0500, Michael S. Tsirkin wrote:
> On Fri, Feb 21, 2020 at 03:33:40PM +0100, Halil Pasic wrote:
> > AFAIU you have a positive attitude towards the idea, that 
> > !F_VIRTIO_PLATFORM implies 'no DMA API is used by virtio' 
> > should be scrapped. 
> > 
> > I would like to accomplish that without adverse effects to virtio-ccw
> > (because caring for virtio-ccw is a part of job description).
> > 
> > Regards,
> > Halil
> 
> It is possible, in theory. IIRC the main challenge is that DMA API
> has overhead of indirect function calls even when all it
> does it return back the PA without changes.

That overhead is gone now, the DMA direct calls are direct calls these
days.