From: Michael Kelley <mhklinux@outlook.com>
To: Yu Zhang <zhangyu1@linux.microsoft.com>,
Michael Kelley <mhklinux@outlook.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
"iommu@lists.linux.dev" <iommu@lists.linux.dev>,
"linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
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"jacob.pan@linux.microsoft.com" <jacob.pan@linux.microsoft.com>,
"tgopinath@linux.microsoft.com" <tgopinath@linux.microsoft.com>,
"easwar.hariharan@linux.microsoft.com"
<easwar.hariharan@linux.microsoft.com>,
Mukesh R <mrathor@linux.microsoft.com>
Subject: RE: [PATCH v1 3/4] iommu/hyperv: Add para-virtualized IOMMU support for Hyper-V guest
Date: Fri, 15 May 2026 17:36:40 +0000 [thread overview]
Message-ID: <SN6PR02MB4157A1B2D9B56062A0917BC5D4042@SN6PR02MB4157.namprd02.prod.outlook.com> (raw)
In-Reply-To: <fw2pruvjgo7yigtcxssf3xv27soibsj6hmw2ls5wj4rylfhdha@e63f32cwu2x5>
From: Yu Zhang <zhangyu1@linux.microsoft.com> Sent: Friday, May 15, 2026 9:54 AM
>
> On Fri, May 15, 2026 at 02:51:38PM +0000, Michael Kelley wrote:
> > From: Yu Zhang <zhangyu1@linux.microsoft.com> Sent: Friday, May 15, 2026 7:00 AM
> > >
> > > On Thu, May 14, 2026 at 06:13:24PM +0000, Michael Kelley wrote:
> > > > From: Yu Zhang <zhangyu1@linux.microsoft.com> Sent: Monday, May 11, 2026 9:24 AM
[....]
> > > >
> > > > Previous versions of this function did hv_iommu_detach_dev(). With that call
> > > > removed from here, hv_iommu_detach_dev() is only called when attaching a
> > > > domain to a device that already has a domain attached. Is it the case that
> > > > Hyper-V doesn't require the detach as a cleanup step?
> > > >
> > >
> > > The IOMMU core attaches the device to release_domain (our blocking domain)
> > > before calling release_device(), so I believe the explicit detach in the RFC
> > > was redundant. I simply didn't realize that at the time.
> > >
> >
> > Got it. But after the IOMMU core attaches the device to the blocking
> > domain, there's the possibility that the vPCI device is rescinded by
> > Hyper-V and it goes away entirely. Or the device might be subjected
> > to an "unbind/bind" cycle in Linux. Does the detach need to be done
> > on the blocking domain in such cases? In this version of the patches, the
> > Hyper-V "attach" and "detach" hypercalls still end up unbalanced. That
> > seems a bit untidy at best, and I wonder if there are scenarios where
> > Hyper-V will complain about the lack of balance.
> >
>
> Thank you, Michael. May I ask what "the vPCI device is rescinded by
> Hyper-V and it goes away entirely" mean?
>
See the documentation at Documentation/virt/hyperv/vpci.rst in a
kernel source code tree, and particularly the section entitled "PCI Device
Removal". Such removals can and do happen in running Azure guest
VMs. Start with that info and then I'll do my best to answer follow-up
questions you may have.
The unbind/bind case is separate, but has some of the same effects in
that Linux should be removing all setup of the PCI device. There's actually
two unbind steps -- one to unbind the device-specific driver (e.g., the
Mellanox MLX5 driver or the NMVe driver) driver from the device, and
potentially a second to unbind the VMBus vPCI driver from the device.
These unbind/bind sequences can be done in the Linux guest without
the Hyper-V host rescinding the device.
Michael
next prev parent reply other threads:[~2026-05-15 17:36 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-11 16:24 [PATCH v1 0/4] Hyper-V: Add para-virtualized IOMMU support for Linux guests Yu Zhang
2026-05-11 16:24 ` [PATCH v1 1/4] iommu: Move Hyper-V IOMMU driver to its own subdirectory Yu Zhang
2026-05-15 22:19 ` Jason Gunthorpe
2026-05-20 6:37 ` Yu Zhang
2026-05-20 13:38 ` Jason Gunthorpe
2026-05-11 16:24 ` [PATCH v1 2/4] hyperv: Introduce new hypercall interfaces used by Hyper-V guest IOMMU Yu Zhang
2026-05-12 21:24 ` sashiko-bot
2026-05-11 16:24 ` [PATCH v1 3/4] iommu/hyperv: Add para-virtualized IOMMU support for Hyper-V guest Yu Zhang
2026-05-12 22:30 ` sashiko-bot
2026-05-13 18:39 ` Jacob Pan
2026-05-15 12:38 ` Yu Zhang
2026-05-14 18:13 ` Michael Kelley
2026-05-15 13:59 ` Yu Zhang
2026-05-15 14:51 ` Michael Kelley
2026-05-15 16:53 ` Yu Zhang
2026-05-15 17:36 ` Michael Kelley [this message]
2026-05-20 15:50 ` Yu Zhang
2026-05-16 0:11 ` Mukesh R
2026-05-18 9:38 ` Yu Zhang
2026-05-15 22:31 ` Jason Gunthorpe
2026-05-20 15:25 ` Yu Zhang
2026-05-20 18:27 ` Jacob Pan
2026-05-21 12:27 ` Yu Zhang
2026-05-11 16:24 ` [PATCH v1 4/4] iommu/hyperv: Add page-selective IOTLB flush support Yu Zhang
2026-05-12 23:45 ` sashiko-bot
2026-05-14 18:14 ` Michael Kelley
2026-05-14 21:16 ` Michael Kelley
2026-05-15 16:23 ` Yu Zhang
2026-05-15 18:00 ` Michael Kelley
2026-05-15 23:33 ` Michael Kelley
2026-05-20 16:27 ` Yu Zhang
2026-05-15 22:35 ` Jason Gunthorpe
2026-05-20 17:15 ` Yu Zhang
2026-05-20 17:29 ` Jason Gunthorpe
2026-05-20 19:26 ` Michael Kelley
2026-05-20 20:40 ` Jacob Pan
2026-05-21 15:45 ` Michael Kelley
2026-05-21 14:34 ` Yu Zhang
2026-05-21 15:39 ` Michael Kelley
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