From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932287AbcBANXg (ORCPT ); Mon, 1 Feb 2016 08:23:36 -0500 Received: from mx1.redhat.com ([209.132.183.28]:49844 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932066AbcBANXe (ORCPT ); Mon, 1 Feb 2016 08:23:34 -0500 Date: Mon, 1 Feb 2016 15:23:24 +0200 From: "Michael S. Tsirkin" To: David Woodhouse Cc: Andy Lutomirski , Benjamin Herrenschmidt , "linux-kernel@vger.kernel.org" , "David S. Miller" , sparclinux@vger.kernel.org, Joerg Roedel , Christian Borntraeger , Cornelia Huck , Sebastian Ott , Paolo Bonzini , Christoph Hellwig , KVM , Martin Schwidefsky , linux-s390 , Linux Virtualization , David Vrabel , Stefano Stabellini , xen-devel@lists.xenproject.org Subject: Re: [PATCH v5 04/10] vring: Introduce vring_use_dma_api() Message-ID: <20160201133556-mutt-send-email-mst@redhat.com> References: <1454325723.133285.285.camel@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1454325723.133285.285.camel@infradead.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > +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. > It would be functionally equivalent, but it would help to push the > workarounds to the right place — rather than entrenching them for ever > in tricky "OMG we need to audit what all the architectures do... let's > not touch it!" code. > > -- > David Woodhouse Open Source Technology Centre > David.Woodhouse@intel.com Intel Corporation >