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

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Message-ID: <6f49be01-89e3-4407-9813-51d62e723947@arm.com>
Date: Thu, 21 Dec 2023 11:01:56 +0000
From: Robin Murphy <robin.murphy@....com>
To: Lukas Wunner <lukas@...ner.de>
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 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.

Thanks,
Robin.