linux-kernel - Re: [PATCH 3/3] virtio

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Date:	Wed, 28 Oct 2015 11:22:52 +0900
From:	David Woodhouse <dwmw2@...radead.org>
To:	Joerg Roedel <jroedel@...e.de>, Andy Lutomirski <luto@...nel.org>
Cc:	linux-kernel@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Christoph Hellwig <hch@....de>, benh@...nel.crashing.org,
	KVM <kvm@...r.kernel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 3/3] virtio_pci: Use the DMA API

On Wed, 2015-10-28 at 11:15 +0900, Joerg Roedel wrote:
> On Tue, Oct 27, 2015 at 06:17:10PM -0700, Andy Lutomirski wrote:
> > From: Andy Lutomirski <luto@...capital.net>
> > 
> > This fixes virtio-pci on platforms and busses that have IOMMUs. 
> >  This
> > will break the experimental QEMU Q35 IOMMU support until QEMU is
> > fixed.  In exchange, it fixes physical virtio hardware as well as
> > virtio-pci running under Xen.
> > 
> > We should clean up the virtqueue API to do its own allocation and
> > teach virtqueue_get_avail and virtqueue_get_used to return DMA
> > addresses directly.
> > 
> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
> > ---
> >  drivers/virtio/virtio_pci_common.h |  3 ++-
> >  drivers/virtio/virtio_pci_legacy.c | 19 +++++++++++++++----
> >  drivers/virtio/virtio_pci_modern.c | 34 ++++++++++++++++++++++++--
> > --------
> >  3 files changed, 41 insertions(+), 15 deletions(-)
> 
> Same here, you need to call the dma_sync* functions when passing data
> from/to the virtio-device.
> 
> I think a good test for that is to boot a virtio kvm-guest with
> swiotlb=force and see if it still works.

That's useful but doesn't cover the cases where dma_wmb() is needed,
right? 

We should make sure we're handling descriptors properly as we would for
real hardware, and ensuring that addresses etc. are written to the
descriptor (and a write barrier occurs) *before* the bit is set which
tells the 'hardware' that it owns that descriptor.

AFAICT we're currently setting the flags *first* in virtqueue_add(),
let alone the missing write barrier between the two.

-- 
dwmw2



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