From: Jean-Philippe Brucker <jean-philippe@linaro.org>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
"Alex Williamson \(alex.williamson@redhat.com\)"
<alex.williamson@redhat.com>, "Raj, Ashok" <ashok.raj@intel.com>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
Jonathan Corbet <corbet@lwn.net>,
Robin Murphy <robin.murphy@arm.com>,
LKML <linux-kernel@vger.kernel.org>,
Kirti Wankhede <kwankhede@nvidia.com>,
"iommu@lists.linux-foundation.org"
<iommu@lists.linux-foundation.org>,
Jason Gunthorpe <jgg@nvidia.com>,
"Jiang, Dave" <dave.jiang@intel.com>,
David Woodhouse <dwmw2@infradead.org>,
Jason Wang <jasowang@redhat.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal
Date: Thu, 10 Jun 2021 18:37:31 +0200 [thread overview]
Message-ID: <YMI/yynDsX/aaG8T@myrica> (raw)
In-Reply-To: <YL8O1pAlg1jtHudn@yekko>
On Tue, Jun 08, 2021 at 04:31:50PM +1000, David Gibson wrote:
> For the qemu case, I would imagine a two stage fallback:
>
> 1) Ask for the exact IOMMU capabilities (including pagetable
> format) that the vIOMMU has. If the host can supply, you're
> good
>
> 2) If not, ask for a kernel managed IOAS. Verify that it can map
> all the IOVA ranges the guest vIOMMU needs, and has an equal or
> smaller pagesize than the guest vIOMMU presents. If so,
> software emulate the vIOMMU by shadowing guest io pagetable
> updates into the kernel managed IOAS.
>
> 3) You're out of luck, don't start.
>
> For both (1) and (2) I'd expect it to be asking this question *after*
> saying what devices are attached to the IOAS, based on the virtual
> hardware configuration. That doesn't cover hotplug, of course, for
> that you have to just fail the hotplug if the new device isn't
> supportable with the IOAS you already have.
Yes. So there is a point in time when the IOAS is frozen, and cannot take
in new incompatible devices. I think that can support the usage I had in
mind. If the VMM (non-QEMU, let's say) wanted to create one IOASID FD per
feature set it could bind the first device, freeze the features, then bind
the second device. If the second bind fails it creates a new FD, allowing
to fall back to (2) for the second device while keeping (1) for the first
device. A paravirtual IOMMU like virtio-iommu could easily support this as
it describes pIOMMU properties for each device to the guest. An emulated
vIOMMU could also support some hybrid cases as you describe below.
> One can imagine optimizations where for certain intermediate cases you
> could do a lighter SW emu if the host supports a model that's close to
> the vIOMMU one, and you're able to trap and emulate the differences.
> In practice I doubt anyone's going to have time to look for such cases
> and implement the logic for it.
>
> > For example depending whether the hardware IOMMU is SMMUv2 or SMMUv3, that
> > completely changes the capabilities offered to the guest (some v2
> > implementations support nesting page tables, but never PASID nor PRI
> > unlike v3.) The same vIOMMU could support either, presenting different
> > capabilities to the guest, even multiple page table formats if we wanted
> > to be exhaustive (SMMUv2 supports the older 32-bit descriptor), but it
> > needs to know early on what the hardware is precisely. Then some new page
> > table format shows up and, although the vIOMMU can support that in
> > addition to older ones, QEMU will have to pick a single one, that it
> > assumes the guest knows how to drive?
> >
> > I think once it binds a device to an IOASID fd, QEMU will want to probe
> > what hardware features are available before going further with the vIOMMU
> > setup (is there PASID, PRI, which page table formats are supported,
> > address size, page granule, etc). Obtaining precise information about the
> > hardware would be less awkward than trying different configurations until
> > one succeeds. Binding an additional device would then fail if its pIOMMU
> > doesn't support exactly the features supported for the first device,
> > because we don't know which ones the guest will choose. QEMU will have to
> > open a new IOASID fd for that device.
>
> No, this fundamentally misunderstands the qemu model. The user
> *chooses* the guest visible platform, and qemu supplies it or fails.
> There is no negotiation with the guest, because this makes managing
> migration impossibly difficult.
I'd like to understand better where the difficulty lies, with migration.
Is the problem, once we have a guest running on physical machine A, to
make sure that physical machine B supports the same IOMMU properties
before migrating the VM over to B? Why can't QEMU (instead of the user)
select a feature set on machine A, then when time comes to migrate, query
all information from the host kernel on machine B and check that it
matches what was picked for machine A? Or is it only trying to
accommodate different sets of features between A and B, that would be too
difficult?
Thanks,
Jean
>
> -cpu host is an exception, which is used because it is so useful, but
> it's kind of a pain on the qemu side. Virt management systems like
> oVirt/RHV almost universally *do not use* -cpu host, precisely because
> it cannot support predictable migration.
>
> --
> David Gibson | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
> | _way_ _around_!
> http://www.ozlabs.org/~dgibson
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next prev parent reply other threads:[~2021-06-10 16:37 UTC|newest]
Thread overview: 260+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-27 7:58 [RFC] /dev/ioasid uAPI proposal Tian, Kevin
2021-05-28 2:24 ` Jason Wang
2021-05-28 20:25 ` Jason Gunthorpe
2021-05-31 8:41 ` Liu Yi L
2021-06-01 2:36 ` Jason Wang
2021-06-01 3:31 ` Liu Yi L
2021-06-01 5:08 ` Jason Wang
2021-06-01 5:23 ` Lu Baolu
2021-06-01 5:29 ` Jason Wang
2021-06-01 5:42 ` Tian, Kevin
2021-06-01 6:07 ` Jason Wang
2021-06-01 6:16 ` Tian, Kevin
2021-06-01 8:47 ` Jason Wang
2021-06-01 17:31 ` Jason Gunthorpe
2021-06-02 8:54 ` Jason Wang
2021-06-02 17:21 ` Jason Gunthorpe
2021-06-07 13:30 ` Enrico Weigelt, metux IT consult
2021-06-07 18:01 ` Jason Gunthorpe
2021-06-08 10:45 ` Enrico Weigelt, metux IT consult
2021-06-10 2:16 ` Jason Wang
2021-06-08 1:10 ` Jason Wang
2021-06-08 13:20 ` Jason Gunthorpe
2021-06-10 2:00 ` Jason Wang
2021-06-10 4:03 ` Jason Wang
2021-06-10 11:47 ` Jason Gunthorpe
2021-06-11 5:43 ` Jason Wang
2021-06-01 17:29 ` Jason Gunthorpe
2021-06-02 8:58 ` Jason Wang
2021-06-01 4:27 ` Shenming Lu
2021-06-01 5:10 ` Jason Wang
2021-05-28 16:23 ` Jean-Philippe Brucker
2021-05-28 20:16 ` Jason Gunthorpe
2021-06-01 7:50 ` Tian, Kevin
2021-05-28 17:35 ` Jason Gunthorpe
2021-06-01 8:10 ` Tian, Kevin
2021-06-01 17:42 ` Jason Gunthorpe
2021-06-02 1:33 ` Tian, Kevin
2021-06-02 16:09 ` Jason Gunthorpe
2021-06-03 1:29 ` Tian, Kevin
2021-06-03 5:09 ` David Gibson
2021-06-03 6:49 ` Tian, Kevin
2021-06-03 11:47 ` Jason Gunthorpe
2021-06-04 2:15 ` Tian, Kevin
2021-06-08 0:49 ` David Gibson
2021-06-09 2:52 ` Tian, Kevin
2021-06-02 6:32 ` David Gibson
2021-06-02 16:16 ` Jason Gunthorpe
2021-06-03 2:11 ` Tian, Kevin
2021-06-03 5:13 ` David Gibson
2021-06-03 11:52 ` Jason Gunthorpe
2021-06-08 0:53 ` David Gibson
2021-06-08 19:04 ` Jason Gunthorpe
2021-06-17 2:42 ` David Gibson
2021-05-28 19:58 ` Jason Gunthorpe
2021-06-01 8:38 ` Tian, Kevin
2021-06-01 17:56 ` Jason Gunthorpe
2021-06-02 2:00 ` Tian, Kevin
2021-06-02 6:57 ` David Gibson
2021-06-02 16:37 ` Jason Gunthorpe
2021-06-03 5:23 ` David Gibson
2021-06-03 12:28 ` Jason Gunthorpe
2021-06-08 6:04 ` David Gibson
2021-06-08 19:23 ` Jason Gunthorpe
2021-06-02 6:48 ` David Gibson
2021-06-02 16:58 ` Jason Gunthorpe
2021-06-03 2:49 ` Tian, Kevin
2021-06-03 5:48 ` David Gibson
2021-06-03 5:45 ` David Gibson
2021-06-03 12:11 ` Jason Gunthorpe
2021-06-04 6:08 ` Tian, Kevin
2021-06-04 12:33 ` Jason Gunthorpe
2021-06-04 23:20 ` Tian, Kevin
2021-06-08 6:13 ` David Gibson
2021-06-04 10:24 ` Jean-Philippe Brucker
2021-06-04 12:05 ` Jason Gunthorpe
2021-06-04 17:27 ` Jacob Pan
2021-06-04 17:40 ` Jason Gunthorpe
2021-06-08 6:31 ` David Gibson
2021-06-10 16:37 ` Jean-Philippe Brucker [this message]
2021-06-17 3:00 ` David Gibson
2021-06-18 17:03 ` Jean-Philippe Brucker
2021-06-18 18:30 ` Jason Gunthorpe
2021-06-23 8:19 ` Tian, Kevin
2021-06-23 7:57 ` Tian, Kevin
2021-06-24 3:49 ` David Gibson
2021-05-28 20:03 ` Jason Gunthorpe
2021-06-01 7:01 ` Tian, Kevin
2021-06-01 20:28 ` Jason Gunthorpe
2021-06-02 1:25 ` Tian, Kevin
2021-06-02 23:27 ` Jason Gunthorpe
2021-06-04 8:17 ` Jean-Philippe Brucker
2021-06-04 8:43 ` Tian, Kevin
2021-06-02 8:52 ` Jason Wang
2021-06-02 16:07 ` Jason Gunthorpe
2021-06-01 22:22 ` Alex Williamson
2021-06-02 2:20 ` Tian, Kevin
2021-06-02 16:01 ` Jason Gunthorpe
2021-06-02 17:11 ` Alex Williamson
2021-06-02 17:35 ` Jason Gunthorpe
2021-06-02 18:01 ` Alex Williamson
2021-06-02 18:09 ` Jason Gunthorpe
2021-06-02 19:00 ` Alex Williamson
2021-06-02 19:54 ` Jason Gunthorpe
2021-06-02 20:37 ` Alex Williamson
2021-06-02 22:45 ` Jason Gunthorpe
2021-06-03 2:50 ` Alex Williamson
2021-06-03 3:22 ` Tian, Kevin
2021-06-03 4:14 ` Alex Williamson
2021-06-03 5:18 ` Tian, Kevin
2021-06-03 12:40 ` Jason Gunthorpe
2021-06-03 20:41 ` Alex Williamson
2021-06-04 9:19 ` Tian, Kevin
2021-06-04 15:37 ` Alex Williamson
2021-06-04 12:13 ` Jason Gunthorpe
2021-06-04 21:45 ` Alex Williamson
2021-06-04 7:33 ` Tian, Kevin
2021-06-03 12:34 ` Jason Gunthorpe
2021-06-03 20:01 ` Alex Williamson
2021-06-03 20:10 ` Jason Gunthorpe
2021-06-03 21:44 ` Alex Williamson
2021-06-04 8:38 ` Tian, Kevin
2021-06-04 12:28 ` Jason Gunthorpe
2021-06-04 15:26 ` Alex Williamson
2021-06-04 15:40 ` Paolo Bonzini
2021-06-04 15:50 ` Jason Gunthorpe
2021-06-04 15:57 ` Paolo Bonzini
2021-06-04 16:03 ` Jason Gunthorpe
2021-06-04 16:10 ` Paolo Bonzini
2021-06-04 17:22 ` Jason Gunthorpe
2021-06-04 21:29 ` Alex Williamson
2021-06-04 23:01 ` Jason Gunthorpe
2021-06-07 15:41 ` Alex Williamson
2021-06-07 18:18 ` Jason Gunthorpe
2021-06-07 18:59 ` Alex Williamson
2021-06-07 19:08 ` Jason Gunthorpe
2021-06-07 19:41 ` Alex Williamson
2021-06-07 23:03 ` Jason Gunthorpe
2021-06-08 0:30 ` Alex Williamson
2021-06-08 1:20 ` Jason Wang
2021-06-30 6:53 ` Christoph Hellwig
2021-06-30 6:49 ` Christoph Hellwig
2021-06-07 3:25 ` Tian, Kevin
2021-06-07 6:51 ` Paolo Bonzini
2021-06-07 18:01 ` Jason Gunthorpe
2021-06-30 6:56 ` Christoph Hellwig
2021-06-05 6:22 ` Paolo Bonzini
2021-06-07 3:50 ` Tian, Kevin
2021-06-07 17:59 ` Jason Gunthorpe
2021-06-08 7:56 ` Paolo Bonzini
2021-06-08 13:15 ` Jason Gunthorpe
2021-06-08 13:44 ` Paolo Bonzini
2021-06-08 18:47 ` Alex Williamson
2021-06-08 19:00 ` Jason Gunthorpe
2021-06-09 8:51 ` Enrico Weigelt, metux IT consult
2021-06-09 9:11 ` Paolo Bonzini
2021-06-09 11:54 ` Jason Gunthorpe
2021-06-09 14:31 ` Alex Williamson
2021-06-09 14:45 ` Jason Gunthorpe
2021-06-09 15:20 ` Paolo Bonzini
2021-10-27 6:18 ` Tian, Kevin
2021-10-27 10:32 ` Paolo Bonzini
2021-10-28 1:50 ` Tian, Kevin
2021-06-09 2:49 ` Tian, Kevin
2021-06-09 11:57 ` Jason Gunthorpe
2021-06-09 12:46 ` Paolo Bonzini
2021-06-09 12:47 ` Jason Gunthorpe
2021-06-09 13:24 ` Paolo Bonzini
2021-06-09 14:32 ` Jason Gunthorpe
2021-06-30 7:01 ` Christoph Hellwig
2021-06-09 18:09 ` Alex Williamson
2021-06-03 2:52 ` Jason Wang
2021-06-03 13:09 ` Jason Gunthorpe
2021-06-04 1:11 ` Jason Wang
2021-06-04 11:58 ` Jason Gunthorpe
2021-06-07 3:18 ` Jason Wang
2021-06-07 14:14 ` Jason Gunthorpe
2021-06-08 1:00 ` Jason Wang
2021-06-08 8:54 ` Enrico Weigelt, metux IT consult
2021-06-08 12:52 ` Jason Gunthorpe
2021-06-30 7:07 ` Christoph Hellwig
2021-06-30 7:05 ` Christoph Hellwig
2021-06-08 2:37 ` David Gibson
2021-06-08 13:17 ` Jason Gunthorpe
2021-06-17 3:47 ` David Gibson
2021-06-23 7:59 ` Tian, Kevin
2021-06-24 3:53 ` David Gibson
2021-05-28 23:36 ` Jason Gunthorpe
2021-05-31 11:31 ` Liu Yi L
2021-05-31 18:09 ` Jason Gunthorpe
2021-06-01 3:08 ` Lu Baolu
2021-06-01 17:24 ` Jason Gunthorpe
2021-06-01 1:25 ` Lu Baolu
2021-06-01 11:09 ` Lu Baolu
2021-06-01 17:26 ` Jason Gunthorpe
2021-06-02 4:01 ` Lu Baolu
2021-06-02 23:23 ` Jason Gunthorpe
2021-06-03 5:49 ` Lu Baolu
2021-06-03 5:54 ` David Gibson
2021-06-03 6:50 ` Lu Baolu
2021-06-03 12:56 ` Jason Gunthorpe
2021-06-02 7:22 ` David Gibson
2021-06-03 6:39 ` Tian, Kevin
2021-06-03 13:05 ` Jason Gunthorpe
2021-06-04 6:37 ` Tian, Kevin
2021-06-04 12:09 ` Jason Gunthorpe
2021-06-04 23:10 ` Tian, Kevin
2021-06-07 17:54 ` Jason Gunthorpe
2021-06-15 8:59 ` Tian, Kevin
2021-06-15 15:06 ` Jason Gunthorpe
2021-06-15 22:59 ` Tian, Kevin
2021-06-15 23:02 ` Jason Gunthorpe
2021-06-15 23:09 ` Tian, Kevin
2021-06-15 23:40 ` Jason Gunthorpe
2021-06-15 23:56 ` Tian, Kevin
2021-06-15 23:59 ` Jason Gunthorpe
2021-06-16 0:02 ` Tian, Kevin
2021-05-31 17:37 ` Parav Pandit
2021-05-31 18:12 ` Jason Gunthorpe
2021-06-01 12:04 ` Parav Pandit
2021-06-01 17:36 ` Jason Gunthorpe
2021-06-02 8:38 ` Enrico Weigelt, metux IT consult
2021-06-02 12:41 ` Parav Pandit
2021-06-01 4:31 ` Shenming Lu
2021-06-01 5:10 ` Lu Baolu
2021-06-01 7:15 ` Shenming Lu
2021-06-01 12:30 ` Lu Baolu
2021-06-01 13:10 ` Shenming Lu
2021-06-01 17:33 ` Jason Gunthorpe
2021-06-02 4:50 ` Shenming Lu
2021-06-03 18:19 ` Jacob Pan
2021-06-04 1:30 ` Jason Wang
2021-06-04 16:22 ` Jacob Pan
2021-06-04 16:22 ` Jason Gunthorpe
2021-06-04 18:05 ` Jacob Pan
2021-06-04 2:03 ` Shenming Lu
2021-06-07 12:19 ` Liu, Yi L
2021-06-08 1:09 ` Shenming Lu
2021-06-01 17:30 ` Parav Pandit
2021-06-03 20:58 ` Jacob Pan
2021-06-08 6:30 ` Parav Pandit
2021-06-02 6:15 ` David Gibson
2021-06-02 17:19 ` Jason Gunthorpe
2021-06-03 3:02 ` Tian, Kevin
2021-06-03 6:26 ` David Gibson
2021-06-03 12:46 ` Jason Gunthorpe
2021-06-04 6:27 ` Tian, Kevin
2021-06-03 7:17 ` Tian, Kevin
2021-06-03 12:49 ` Jason Gunthorpe
2021-06-08 5:49 ` David Gibson
2021-06-03 8:12 ` Tian, Kevin
2021-06-17 4:07 ` David Gibson
2021-06-23 8:00 ` Tian, Kevin
2021-06-24 3:55 ` David Gibson
2021-06-02 8:56 ` Enrico Weigelt, metux IT consult
2021-06-02 17:24 ` Jason Gunthorpe
2021-06-04 10:44 ` Enrico Weigelt, metux IT consult
2021-06-04 12:30 ` Jason Gunthorpe
2021-06-08 1:15 ` David Gibson
2021-06-08 10:43 ` Enrico Weigelt, metux IT consult
2021-06-08 13:11 ` Jason Gunthorpe
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