From: "Michael S. Tsirkin" <mst@redhat.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>,
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, benh@kernel.crashing.org,
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: Mon, 30 Jul 2018 16:26:32 +0300 [thread overview]
Message-ID: <20180730155633-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20180730111802.GA9830@infradead.org>
On Mon, Jul 30, 2018 at 04:18:02AM -0700, Christoph Hellwig wrote:
> On Mon, Jul 30, 2018 at 01:28:03PM +0300, Michael S. Tsirkin wrote:
> > Let me reply to the "crappy" part first:
> > So virtio devices can run on another CPU or on a PCI bus. Configuration
> > can happen over mupltiple transports. There is a discovery protocol to
> > figure out where it is. It has some warts but any real system has warts.
> >
> > So IMHO virtio running on another CPU isn't "legacy virtual crappy
> > virtio". virtio devices that actually sit on a PCI bus aren't "sane"
> > simply because the DMA is more convoluted on some architectures.
>
> All of what you said would be true if virtio didn't claim to be
> a PCI device.
There's nothing virtio claims to be. It's a PV device that uses PCI for
its configuration. Configuration is enumerated on the virtual PCI bus.
That part of the interface is emulated PCI. Data path is through a
PV device enumerated on the virtio bus.
> Once it claims to be a PCI device and we also see
> real hardware written to the interface I stand to all what I said
> above.
Real hardware would reuse parts of the interface but by necessity it
needs to behave slightly differently on some platforms. However for
some platforms (such as x86) a PV virtio driver will by luck work with a
PCI device backend without changes. As these platforms and drivers are
widely deployed, some people will deploy hardware like that. Should be
a non issue as by definition it's transparent to guests.
> > With this out of my system:
> > I agree these approaches are hacky. I think it is generally better to
> > have virtio feature negotiation tell you whether device runs on a CPU or
> > not rather than rely on platform specific ways for this. To this end
> > there was a recent proposal to rename VIRTIO_F_IO_BARRIER to
> > VIRTIO_F_REAL_DEVICE. It got stuck since "real" sounds vague to people,
> > e.g. what if it's a VF - is that real or not? But I can see something
> > like e.g. VIRTIO_F_PLATFORM_DMA gaining support.
> >
> > We would then rename virtio_has_iommu_quirk to virtio_has_dma_quirk
> > and test VIRTIO_F_PLATFORM_DMA in addition to the IOMMU thing.
>
> I don't really care about the exact naming, and indeed a device that
> sets the flag doesn't have to be a 'real' device - it just has to act
> like one. I explained all the issues that this means (at least relating
> to DMA) in one of the previous threads.
I believe you refer to this:
https://lkml.org/lkml/2018/6/7/15
that was a very helpful list outlining the problems we need to solve,
thanks a lot for that!
> The important bit is that we can specify exact behavior for both
> devices that sets the "I'm real!" flag and that ones that don't exactly
> in the spec.
I would very much like that, yes.
> And that very much excludes arch-specific (or
> Xen-specific) overrides.
We already committed to a xen specific hack but generally I prefer
devices that describe how they work instead of platforms magically
guessing, yes.
However the question people raise is that DMA API is already full of
arch-specific tricks the likes of which are outlined in your post linked
above. How is this one much worse?
--
MST
next prev parent reply other threads:[~2018-07-30 13:26 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 [this message]
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
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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