From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH v5 04/10] vring: Introduce vring_use_dma_api() Date: Mon, 1 Feb 2016 07:39:31 -0800 Message-ID: References: <1454325723.133285.285.camel@infradead.org> <20160201133556-mutt-send-email-mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20160201133556-mutt-send-email-mst@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: "Michael S. Tsirkin" Cc: Joerg Roedel , KVM , linux-s390 , Benjamin Herrenschmidt , Stefano Stabellini , Sebastian Ott , "linux-kernel@vger.kernel.org" , Christoph Hellwig , Christian Borntraeger , David Vrabel , Andy Lutomirski , "xen-devel@lists.xenproject.org" , sparclinux@vger.kernel.org, Paolo Bonzini , Linux Virtualization , David Woodhouse , "David S. Miller" , Martin Schwidefsky List-Id: virtualization@lists.linuxfoundation.org On Mon, Feb 1, 2016 at 5:23 AM, Michael S. Tsirkin wrote: > On Mon, Feb 01, 2016 at 11:22:03AM +0000, David Woodhouse wrote: >> On Thu, 2016-01-28 at 18:31 -0800, Andy Lutomirski wrote: >> > This is a kludge, but no one has come up with a a better idea yet. >> > We'll introduce DMA API support guarded by vring_use_dma_api(). >> > Eventually we may be able to return true on more and more systems, >> > and hopefully we can get rid of vring_use_dma_api() entirely some >> > day. >> > >> > Signed-off-by: Andy Lutomirski >> > --- >> > drivers/virtio/virtio_ring.c | 24 ++++++++++++++++++++++++ >> > 1 file changed, 24 insertions(+) >> > >> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c >> > index e12e385f7ac3..4b8dab4960bb 100644 >> > --- a/drivers/virtio/virtio_ring.c >> > +++ b/drivers/virtio/virtio_ring.c >> > @@ -25,6 +25,30 @@ >> > #include >> > #include >> > >> > +/* >> > + * The interaction between virtio and a possible IOMMU is a mess. >> > + * >> > + * On most systems with virtio, physical addresses match bus addresses, >> > + * and it doesn't particularly matter whether we use the DMI API. >> > + * >> > + * On some sytems, including Xen and any system with a physical device >> > + * that speaks virtio behind a physical IOMMU, we must use the DMA API >> > + * for virtio DMA to work at all. >> > + * >> > + * On other systems, including SPARC and PPC64, virtio-pci devices are >> > + * enumerated as though they are behind an IOMMU, but the virtio host >> > + * ignores the IOMMU, so we must either pretend that the IOMMU isn't >> > + * there or somehow map everything as the identity. >> > + * >> > + * For the time being, we preseve historic behavior and bypass the DMA >> > + * API. >> > + */ >> >> I spot at least three typos in there, FWIW. ('DMI API', 'sytems', >> 'preseve'). > > Good catch, hopefully will be fixed in v2. Queued for v2. > >> > +static bool vring_use_dma_api(void) >> > +{ >> > + return false; >> > +} >> > + >> >> I'd quite like to see this be an explicit opt-out for the known-broken >> platforms. We've listed the SPARC and PPC64 issues. For x86 I need to >> refresh my memory as a prelude to trying to fix it... was the issue >> *just* that Qemu tends to ship with a broken BIOS that misdescribes the >> virtio devices (and any assigned PCI devices) as being behind an IOMMU >> when they're not, in the rare case that Qemu actually exposes its >> partially-implemented virtual IOMMU to the guest? >> >> Could we have an arch_vring_eschew_dma_api(dev) function which the >> affected architectures could provide (as a prelude to fixing it so that >> the DMA API does the right thing for *itself*)? > > I'm fine with this. I modified vring_use_dma_api to take a vring_virtqueue* parameter to make this easier. I'm a bit torn here. I want to get the mechanism and the Xen part in, and there's unlikely to be much debate on those as a matter of principle. I'd also like to flip as many arches over as possible, but that could be trickier. Let me mull over this. --Andy