linux-kernel - Re: [PATCH 2/2] iommu/vt-d: don's issue devTLB flush request when device is disconnected

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Date: Thu, 21 Dec 2023 12:07:43 +0100
From: Lukas Wunner <lukas@...ner.de>
To: Robin Murphy <robin.murphy@....com>
Cc: Ethan Zhao <haifeng.zhao@...ux.intel.com>, bhelgaas@...gle.com,
	baolu.lu@...ux.intel.com, dwmw2@...radead.org, will@...nel.org,
	linux-pci@...r.kernel.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, Haorong Ye <yehaorong@...edance.com>
Subject: Re: [PATCH 2/2] iommu/vt-d: don's issue devTLB flush request when
 device is disconnected

On Thu, Dec 21, 2023 at 11:01:56AM +0000, Robin Murphy wrote:
> On 2023-12-21 10:42 am, Lukas Wunner wrote:
> > On Wed, Dec 13, 2023 at 11:54:05AM +0000, Robin Murphy wrote:
> > > I think if we want to ensure ATCs are invalidated on hot-unplug we need an
> > > additional pre-removal notifier to take care of that, and that step would
> > > then want to distinguish between an orderly removal where cleaning up is
> > > somewhat meaningful, and a surprise removal where it definitely isn't.
> > 
> > Even if a user starts the process for orderly removal, the device may be
> > surprise-removed *during* that process.  So we cannot assume that the
> > device is actually accessible if orderly removal has been initiated.
> > If the form factor supports surprise removal, the device may be gone
> > at any time.
> 
> Sure, whatever we do there's always going to be some unavoidable
> time-of-check-to-time-of-use race window so we can never guarantee that
> sending a request to the device will succeed. I was just making the point
> that if we *have* already detected a surprise removal, then cleaning up its
> leftover driver model state should still generate a BUS_NOTIFY_REMOVE_DEVICE
> call, but in that case we can know there's no point trying to send any
> requests to the device that's already gone.

Right, using pci_dev_is_disconnected() as a *speedup* when we
definitely know the device is gone, that's perfectly fine.

So in that sense the proposed patch is acceptable *after* this
series has been extended to make sure hard lockups can *never*
occur on unplug.

Thanks,

Lukas