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 10:08:24 +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?

It is not about Intel vt-d's capability per my understanding, it is the 
third

party PCIe switch's capability, they are not aware of  ATS transation at 
all,

if its downstream port endpoint device is removed/powered-off/link-down,

it couldn't feedback the upstream iommu a fault/completion/timeout for

ATS transaction breakage reason.  While the root port could (verified).

>
> Granted, the Implementation Note you've pointed to allows 1 sec + 50%,
  1 min (60 sec)+50%
> but that's not even a "must", it's a "should".  So devices are free to

I could happen if blindly wait here, so we should avoid such case.


Thanks,

Ethan

> take even longer.  We have to cut off at *some* point.
>
> Thanks,
>
> Lukas