Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
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>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        linux-s390@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Viktor Mihajlovski <mihajlov@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Ram Pai <linuxram@us.ibm.com>,
        Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        David Gibson <david@gibson.dropbear.id.au>,
        "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
Message-ID: <20200224171657.GB7278@lst.de>
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>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200222140408-mutt-send-email-mst@kernel.org>
User-Agent: Mutt/1.5.17 (2007-11-01)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.