linux-kernel - Re: [RFC PATCH v8 4/5] iommu/vt-d: don't issue device-TLB invalidate request when device is disconnected

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Date: Wed, 27 Dec 2023 07:11:51 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Ethan Zhao <haifeng.zhao@...ux.intel.com>
Cc: bhelgaas@...gle.com, baolu.lu@...ux.intel.com, dwmw2@...radead.org,
	will@...nel.org, robin.murphy@....com, lukas@...ner.de,
	linux-pci@...r.kernel.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v8 4/5] iommu/vt-d: don't issue device-TLB invalidate
 request when device is disconnected

I suggest using "ATS Invalidate Request" in the subject as well.
Otherwise we have to figure out whether "device-TLB invalidate
request" is the same as "ATS Invalidate Request".

If they are the same, just use the same words.

On Tue, Dec 26, 2023 at 09:59:22PM -0500, Ethan Zhao wrote:
> Except those aggressive hotplug cases - surprise remove a hotplug device
> while its safe removal is requested and handled in process by:
> 
> 1. pull it out directly.
> 2. turn off its power.
> 3. bring the link down.
> 4. just died there that moment.
> 
> etc, in a word, 'gone' or 'disconnected'.
> 
> Mostly are regular normal safe removal and surprise removal unplug.
> these hot unplug handling process could be optimized for fix the ATS
> invalidation hang issue by calling pci_dev_is_disconnected() in function
> devtlb_invalidation_with_pasid() to check target device state to avoid
> sending meaningless ATS invalidation request to iommu when device is gone.
> (see IMPLEMENTATION NOTE in PCIe spec r6.1 section 10.3.1)

Suggest "ATS Invalidate Request", capitalized exactly that way so we
know it's a specific name of something defined in the PCIe spec.

> For safe removal, device wouldn't be removed untill the whole software
> handling process is done, it wouldn't trigger the hard lock up issue
> caused by too long ATS invalidation timeout wait. in safe removal path,

Ditto.

Capitalize "In the safe removal ..." since it starts a new sentence.

> device state isn't set to pci_channel_io_perm_failure in
> pciehp_unconfigure_device() by checking 'presence' parameter, calling
> pci_dev_is_disconnected() in devtlb_invalidation_with_pasid() will return
> false there, wouldn't break the function.
> 
> For surprise removal, device state is set to pci_channel_io_perm_failure in
> pciehp_unconfigure_device(), means device is already gone (disconnected)
> call pci_dev_is_disconnected() in devtlb_invalidation_with_pasid() will
> return true to break the function not to send ATS invalidation request to

Ditto.

> the disconnected device blindly, thus avoid the further long time waiting
> triggers the hard lockup.
> 
> safe removal & surprise removal
> 
> pciehp_ist()
>    pciehp_handle_presence_or_link_change()
>      pciehp_disable_slot()
>        remove_board()
>          pciehp_unconfigure_device(presence)