linux-kernel - Re: [PATCH v4 2/2] iommu/vt-d: don's issue devTLB flush request when device is disconnected

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Date: Fri, 22 Dec 2023 11:56:31 +0800
From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
To: Lukas Wunner <lukas@...ner.de>
Cc: bhelgaas@...gle.com, baolu.lu@...ux.intel.com, dwmw2@...radead.org,
 will@...nel.org, robin.murphy@....com, linux-pci@...r.kernel.org,
 iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] iommu/vt-d: don's issue devTLB flush request when
 device is disconnected


On 12/21/2023 7:01 PM, Lukas Wunner wrote:
> On Thu, Dec 21, 2023 at 11:39:40AM +0100, Lukas Wunner wrote:
>> On Tue, Dec 19, 2023 at 07:51:53PM -0500, Ethan Zhao wrote:
>>> For those endpoint devices connect to system via hotplug capable ports,
>>> users could request a warm reset to the device by flapping device's link
>>> through setting the slot's link control register, as pciehpt_ist() DLLSC
>>> interrupt sequence response, pciehp will unload the device driver and
>>> then power it off. thus cause an IOMMU devTLB flush request for device to
>>> be sent and a long time completion/timeout waiting in interrupt context.
>> I think the problem is in the "waiting in interrupt context".
> I'm wondering whether Intel IOMMUs possibly have a (perhaps undocumented)
> capability to reduce the Invalidate Completion Timeout to a sane value?
> Could you check whether that's supported?
>
> Granted, the Implementation Note you've pointed to allows 1 sec + 50%,
> but that's not even a "must", it's a "should".  So devices are free to
> take even longer.  We have to cut off at *some* point.

I really "expected" there is interrrupt signal to iommu hardware when

the PCIe swtich downstream device 'gone', or some internal polling

/heartbeating the endpoint device for ATS breaking,

but so far seems there are only hotplug interrupts to downstream

control.

...

How to define the point "some" msec to timeout while software

break out the waiting loop ?  or polling if the target is gone ?


Thanks,

Ethan



>
> Thanks,
>
> Lukas