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:   Thu, 5 Apr 2018 21:34:54 +0300
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc:     Anshuman Khandual <khandual@linux.vnet.ibm.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, robh@kernel.org, aik@ozlabs.ru,
        jasowang@redhat.com, joe@perches.com,
        linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net,
        david@gibson.dropbear.id.au
Subject: Re: [RFC] virtio: Use DMA MAP API for devices without an IOMMU
Message-ID: <20180405213042-mutt-send-email-mst@kernel.org>
References: <20180405105631.9514-1-khandual@linux.vnet.ibm.com>
 <3e1b113b-79ca-b700-5be9-10c66d74aabe@linux.vnet.ibm.com>
 <20180405175326-mutt-send-email-mst@kernel.org>
 <1522940983.21446.205.camel@kernel.crashing.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1522940983.21446.205.camel@kernel.crashing.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Apr 06, 2018 at 01:09:43AM +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2018-04-05 at 17:54 +0300, Michael S. Tsirkin wrote:
> > On Thu, Apr 05, 2018 at 08:09:30PM +0530, Anshuman Khandual wrote:
> > > On 04/05/2018 04:26 PM, Anshuman Khandual wrote:
> > > > There are certian platforms which would like to use SWIOTLB based DMA API
> > > > for bouncing purpose without actually requiring an IOMMU back end. But the
> > > > virtio core does not allow such mechanism. Right now DMA MAP API is only
> > > > selected for devices which have an IOMMU and then the QEMU/host back end
> > > > will process all incoming SG buffer addresses as IOVA instead of simple
> > > > GPA which is the case for simple bounce buffers after being processed with
> > > > SWIOTLB API. To enable this usage, it introduces an architecture specific
> > > > function which will just make virtio core front end select DMA operations
> > > > structure.
> > > > 
> > > > Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
> > > 
> > > + "Michael S. Tsirkin" <mst@redhat.com>
> > 
> > I'm confused by this.
> > 
> > static bool vring_use_dma_api(struct virtio_device *vdev)
> > {
> >         if (!virtio_has_iommu_quirk(vdev))
> >                 return true;
> > 
> > 
> > Why doesn't setting VIRTIO_F_IOMMU_PLATFORM on the
> > hypervisor side sufficient?
> 
> In this specific case, because that would make qemu expect an iommu,
> and there isn't one.


I think that you can set iommu_platform in qemu without an iommu.


> Anshuman, you need to provide more background here. I don't have time
> right now it's late, but explain about the fact that this is for a
> specific type of secure VM which has only a limited pool of (insecure)
> memory that can be shared with qemu, so all IOs need to bounce via that
> pool, which can be achieved by using swiotlb.
> 
> Note: this isn't urgent, we can discuss alternative approaches, this is
> just to start the conversation.
> 
> Cheers,
> Ben.