From: "Michael S. Tsirkin" <mst@redhat.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Christoph Hellwig <hch@infradead.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Anshuman Khandual <khandual@linux.vnet.ibm.com>,
virtualization@lists.linux-foundation.org,
linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
aik@ozlabs.ru, robh@kernel.org, joe@perches.com,
elfring@users.sourceforge.net, david@gibson.dropbear.id.au,
jasowang@redhat.com, mpe@ellerman.id.au, linuxram@us.ibm.com,
haren@linux.vnet.ibm.com, paulus@samba.org,
srikar@linux.vnet.ibm.com, robin.murphy@arm.com,
jean-philippe.brucker@arm.com, marc.zyngier@arm.com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices
Date: Thu, 2 Aug 2018 01:41:20 +0300 [thread overview]
Message-ID: <20180802014028-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20180801090535.GB14438@arm.com>
On Wed, Aug 01, 2018 at 10:05:35AM +0100, Will Deacon wrote:
> Hi Christoph,
>
> On Wed, Aug 01, 2018 at 01:36:39AM -0700, Christoph Hellwig wrote:
> > On Wed, Aug 01, 2018 at 09:16:38AM +0100, Will Deacon wrote:
> > > On arm/arm64, the problem we have is that legacy virtio devices on the MMIO
> > > transport (so definitely not PCI) have historically been advertised by qemu
> > > as not being cache coherent, but because the virtio core has bypassed DMA
> > > ops then everything has happened to work. If we blindly enable the arch DMA
> > > ops,
> >
> > No one is suggesting that as far as I can tell.
>
> Apologies: it's me that wants the DMA ops enabled to handle legacy devices
> behind an IOMMU, but see below.
>
> > > we'll plumb in the non-coherent ops and start getting data corruption,
> > > so we do need a way to quirk virtio as being "always coherent" if we want to
> > > use the DMA ops (which we do, because our emulation platforms have an IOMMU
> > > for all virtio devices).
> >
> > From all that I've gather so far: no you do not want that. We really
> > need to figure out virtio "dma" interacts with the host / device.
> >
> > If you look at the current iommu spec it does talk of physical address
> > with a little careveout for VIRTIO_F_IOMMU_PLATFORM.
>
> That's true, although that doesn't exist in the legacy virtio spec, and we
> have an existing emulation platform which puts legacy virtio devices behind
> an IOMMU. Currently, Linux is unable to boot on this platform unless the
> IOMMU is configured as bypass. If we can use the coherent IOMMU DMA ops,
> then it works perfectly.
>
> > So between that and our discussion in this thread and its previous
> > iterations I think we need to stick to the current always physical,
> > bypass system dma ops mode of virtio operation as the default.
>
> As above -- that means we hang during boot because we get stuck trying to
> bring up a virtio-block device whose DMA is aborted by the IOMMU. The easy
> answer is "just upgrade to latest virtio and advertise the presence of the
> IOMMU". I'm pushing for that in future platforms, but it seems a shame not
> to support the current platform, especially given that other systems do have
> hacks in mainline to get virtio working.
>
> > We just need to figure out how to deal with devices that deviate
> > from the default. One things is that VIRTIO_F_IOMMU_PLATFORM really
> > should become VIRTIO_F_PLATFORM_DMA to cover the cases of non-iommu
> > dma tweaks (offsets, cache flushing), which seems well in spirit of
> > the original design. The other issue is VIRTIO_F_IO_BARRIER
> > which is very vaguely defined, and which needs a better definition.
> > And last but not least we'll need some text explaining the challenges
> > of hardware devices - I think VIRTIO_F_PLATFORM_DMA + VIRTIO_F_IO_BARRIER
> > is what would basically cover them, but a good description including
> > an explanation of why these matter.
>
> I agree that this makes sense for future revisions of virtio (or perhaps
> it can just be a clarification to virtio 1.0), but we're still left in the
> dark with legacy devices and it would be nice to have them work on the
> systems which currently exist, even if it's a legacy-only hack in the arch
> code.
>
> Will
Myself I'm sympathetic to this use-case and I see more uses to this
than just legacy support. But more work is required IMHO.
Will post tomorrow though - it's late here ...
--
MST
next prev parent reply other threads:[~2018-08-01 22:41 UTC|newest]
Thread overview: 119+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-20 3:59 [RFC 0/4] Virtio uses DMA API for all devices Anshuman Khandual
2018-07-20 3:59 ` [RFC 1/4] virtio: Define virtio_direct_dma_ops structure Anshuman Khandual
2018-07-30 9:24 ` Christoph Hellwig
2018-07-31 4:01 ` Anshuman Khandual
2018-07-20 3:59 ` [RFC 2/4] virtio: Override device's DMA OPS with virtio_direct_dma_ops selectively Anshuman Khandual
2018-07-28 8:56 ` Anshuman Khandual
2018-07-28 21:16 ` Michael S. Tsirkin
2018-07-30 4:15 ` Anshuman Khandual
2018-07-30 9:30 ` Christoph Hellwig
2018-07-31 6:39 ` Anshuman Khandual
2018-07-30 9:25 ` Christoph Hellwig
2018-07-31 7:00 ` Anshuman Khandual
2018-07-20 3:59 ` [RFC 3/4] virtio: Force virtio core to use DMA API callbacks for all virtio devices Anshuman Khandual
2018-07-20 3:59 ` [RFC 4/4] virtio: Add platform specific DMA API translation for virito devices Anshuman Khandual
2018-07-20 13:15 ` Michael S. Tsirkin
2018-07-23 2:16 ` Anshuman Khandual
2018-07-25 4:30 ` Anshuman Khandual
2018-07-25 13:31 ` Michael S. Tsirkin
2018-07-20 13:16 ` [RFC 0/4] Virtio uses DMA API for all devices Michael S. Tsirkin
2018-07-23 6:28 ` Anshuman Khandual
2018-07-23 9:08 ` Michael S. Tsirkin
2018-07-25 3:26 ` Anshuman Khandual
2018-07-27 11:31 ` Michael S. Tsirkin
2018-07-28 8:37 ` Anshuman Khandual
2018-07-27 9:58 ` Will Deacon
2018-07-27 10:58 ` Anshuman Khandual
2018-07-30 9:34 ` Christoph Hellwig
2018-07-30 10:28 ` Michael S. Tsirkin
2018-07-30 11:18 ` Christoph Hellwig
2018-07-30 13:26 ` Michael S. Tsirkin
2018-07-31 17:30 ` Christoph Hellwig
2018-07-31 20:36 ` Benjamin Herrenschmidt
2018-08-01 8:16 ` Will Deacon
2018-08-01 8:36 ` Christoph Hellwig
2018-08-01 9:05 ` Will Deacon
2018-08-01 22:41 ` Michael S. Tsirkin [this message]
2018-08-01 22:35 ` Michael S. Tsirkin
2018-08-02 15:24 ` Benjamin Herrenschmidt
2018-08-02 15:41 ` Michael S. Tsirkin
2018-08-02 16:01 ` Benjamin Herrenschmidt
2018-08-02 17:19 ` Michael S. Tsirkin
2018-08-02 17:53 ` Benjamin Herrenschmidt
2018-08-02 20:52 ` Michael S. Tsirkin
2018-08-02 21:13 ` Benjamin Herrenschmidt
2018-08-02 21:51 ` Michael S. Tsirkin
2018-08-03 7:05 ` Christoph Hellwig
2018-08-03 15:58 ` Benjamin Herrenschmidt
2018-08-03 16:02 ` Christoph Hellwig
2018-08-03 18:58 ` Benjamin Herrenschmidt
2018-08-04 8:21 ` Christoph Hellwig
2018-08-05 1:10 ` Benjamin Herrenschmidt
2018-08-05 7:29 ` Christoph Hellwig
2018-08-05 21:16 ` Benjamin Herrenschmidt
2018-08-05 21:30 ` Benjamin Herrenschmidt
2018-08-06 9:42 ` Christoph Hellwig
2018-08-06 19:52 ` Benjamin Herrenschmidt
2018-08-07 6:21 ` Christoph Hellwig
2018-08-07 6:42 ` Benjamin Herrenschmidt
2018-08-07 13:55 ` Christoph Hellwig
2018-08-07 20:32 ` Benjamin Herrenschmidt
2018-08-08 6:31 ` Christoph Hellwig
2018-08-08 10:07 ` Benjamin Herrenschmidt
2018-08-08 12:30 ` Christoph Hellwig
2018-08-08 13:18 ` Benjamin Herrenschmidt
2018-08-08 20:31 ` Michael S. Tsirkin
2018-08-08 22:13 ` Benjamin Herrenschmidt
2018-08-09 2:00 ` Benjamin Herrenschmidt
2018-08-09 5:40 ` Christoph Hellwig
2018-09-07 0:09 ` Jiandi An
2018-09-10 6:19 ` Christoph Hellwig
2018-09-10 8:53 ` Gerd Hoffmann
2018-08-03 19:07 ` Michael S. Tsirkin
2018-08-04 1:11 ` Benjamin Herrenschmidt
2018-08-04 1:16 ` Benjamin Herrenschmidt
2018-08-05 0:22 ` Michael S. Tsirkin
2018-08-05 4:52 ` Benjamin Herrenschmidt
2018-08-06 13:46 ` Michael S. Tsirkin
2018-08-06 19:56 ` Benjamin Herrenschmidt
2018-08-06 20:35 ` Michael S. Tsirkin
2018-08-06 21:26 ` Benjamin Herrenschmidt
2018-08-06 21:46 ` Michael S. Tsirkin
2018-08-06 22:13 ` Benjamin Herrenschmidt
2018-08-06 23:16 ` Benjamin Herrenschmidt
2018-08-06 23:45 ` Michael S. Tsirkin
2018-08-07 0:18 ` Benjamin Herrenschmidt
2018-08-07 6:32 ` Christoph Hellwig
2018-08-07 6:27 ` Christoph Hellwig
2018-08-07 6:44 ` Benjamin Herrenschmidt
2018-08-07 6:18 ` Christoph Hellwig
2018-08-07 6:16 ` Christoph Hellwig
2018-08-06 23:18 ` Benjamin Herrenschmidt
2018-08-07 6:12 ` Christoph Hellwig
2018-08-04 1:18 ` Benjamin Herrenschmidt
2018-08-04 1:22 ` Benjamin Herrenschmidt
2018-08-05 0:23 ` Michael S. Tsirkin
2018-08-03 19:17 ` Michael S. Tsirkin
2018-08-04 8:15 ` Christoph Hellwig
2018-08-05 0:09 ` Michael S. Tsirkin
2018-08-05 1:11 ` Benjamin Herrenschmidt
2018-08-05 7:25 ` Christoph Hellwig
2018-08-05 0:53 ` Benjamin Herrenschmidt
2018-08-05 0:27 ` Michael S. Tsirkin
2018-08-06 14:05 ` Will Deacon
2018-08-01 21:56 ` Michael S. Tsirkin
2018-08-02 15:33 ` Benjamin Herrenschmidt
2018-08-02 20:53 ` Michael S. Tsirkin
2018-08-03 7:06 ` Christoph Hellwig
2018-08-02 20:55 ` Michael S. Tsirkin
2018-08-03 2:41 ` Jason Wang
2018-08-03 19:08 ` Michael S. Tsirkin
2018-08-04 1:21 ` Benjamin Herrenschmidt
2018-08-05 0:24 ` Michael S. Tsirkin
2018-08-06 9:02 ` Anshuman Khandual
2018-08-06 13:36 ` Michael S. Tsirkin
2018-08-06 15:24 ` Christoph Hellwig
2018-08-06 16:06 ` Michael S. Tsirkin
2018-08-06 16:10 ` Christoph Hellwig
2018-08-06 16:13 ` Michael S. Tsirkin
2018-08-06 16:34 ` Christoph Hellwig
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