linux-kernel - Re: [PATCH v2 10/22] vfio/pci: Skip reset of preserved device after Live Update

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Message-ID: <aXvgKRrbfymW5NKb@google.com>
Date: Thu, 29 Jan 2026 22:33:13 +0000
From: David Matlack <dmatlack@...gle.com>
To: Jacob Pan <jacob.pan@...ux.microsoft.com>
Cc: Alex Williamson <alex@...zbot.org>,
	Adithya Jayachandran <ajayachandra@...dia.com>,
	Alexander Graf <graf@...zon.com>, Alex Mastro <amastro@...com>,
	Alistair Popple <apopple@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ankit Agrawal <ankita@...dia.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, Chris Li <chrisl@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Jason Gunthorpe <jgg@...dia.com>, Jason Gunthorpe <jgg@...pe.ca>,
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	Kevin Tian <kevin.tian@...el.com>, kexec@...ts.infradead.org,
	kvm@...r.kernel.org, Leon Romanovsky <leon@...nel.org>,
	Leon Romanovsky <leonro@...dia.com>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-mm@...ck.org, linux-pci@...r.kernel.org,
	Lukas Wunner <lukas@...ner.de>,
	Michał Winiarski <michal.winiarski@...el.com>,
	Mike Rapoport <rppt@...nel.org>, Parav Pandit <parav@...dia.com>,
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	Zhu Yanjun <yanjun.zhu@...ux.dev>
Subject: Re: [PATCH v2 10/22] vfio/pci: Skip reset of preserved device after
 Live Update

On 2026-01-29 02:21 PM, Jacob Pan wrote:
> On Thu, 29 Jan 2026 21:24:57 +0000 David Matlack <dmatlack@...gle.com> wrote:

> > diff --git a/drivers/vfio/pci/vfio_pci_core.c
> > b/drivers/vfio/pci/vfio_pci_core.c index b01b94d81e28..c9f73f597797
> > 100644 --- a/drivers/vfio/pci/vfio_pci_core.c
> > +++ b/drivers/vfio/pci/vfio_pci_core.c
> > @@ -515,12 +515,24 @@ int vfio_pci_core_enable(struct
> > vfio_pci_core_device *vdev) if (ret)
> >  		goto out_power;
> >  
> > -	/* If reset fails because of the device lock, fail this path
> > entirely */
> > -	ret = pci_try_reset_function(pdev);
> > -	if (ret == -EAGAIN)
> > -		goto out_disable_device;
> > +	if (vdev->liveupdate_incoming_state) {
> > +		/*
> > +		 * This device was preserved by the previous kernel
> > across a
> > +		 * Live Update, so it does not need to be reset.
> > +		 */
> > +		vdev->reset_works =
> > vdev->liveupdate_incoming_state->reset_works;
>
> Just wondering what happened to skipping the bus master clearing. I
> understand this version does not preserve the device itself yet; I’m
> just curious whether there were specific difficulties that led to
> dropping the earlier patch which skipped clearing bus master.

Hi Jacob,

There's several places where bus master gets cleared that we need to
eventually eliminate to fully preserve the device.

 1. vfio_pci_liveupdate_freeze() clears it during shutdown when it
    restores vdev->pci_saved_state.
 2. pci_device_shutdown() clears it during shutdown.
 3. vfio_pci_core_enable() clears it when the preserved device file
    is bound to an iommufd after the Live Update (in
    vfio_pci_core_enable()).

I think it would be safe to skip (3) in this series, since that's very
similar to how this series skips resets during vfio_pci_core_enable()
for preserved devices.

But I don't think it would be safe to skip (1) or (2) until the attached
iommufd is fully preserved.

If you are just asking about (3) then I agree it could be skipped and I
can include that in the next version.