linux-kernel - Re: [RFC PATCH v6 4/4] iommu/vt-d: break out devTLB invalidation if target device is gone

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Date: Mon, 25 Dec 2023 16:57:31 +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: [RFC PATCH v6 4/4] iommu/vt-d: break out devTLB invalidation if
 target device is gone


On 12/24/2023 6:47 PM, Lukas Wunner wrote:
> On Sun, Dec 24, 2023 at 12:06:57AM -0500, Ethan Zhao wrote:
>> --- a/drivers/iommu/intel/dmar.c
>> +++ b/drivers/iommu/intel/dmar.c
>> @@ -1423,6 +1423,13 @@ int qi_submit_sync(struct intel_iommu *iommu, struct qi_desc *desc,
>>   	writel(qi->free_head << shift, iommu->reg + DMAR_IQT_REG);
>>   
>>   	while (qi->desc_status[wait_index] != QI_DONE) {
>> +		/*
>> +		 * if the devTLB invalidation target device is gone, don't wait
>> +		 * anymore, it might take up to 1min+50%, causes system hang.
>> +		 */
>> +		if (type == QI_DIOTLB_TYPE && iommu->flush_target_dev)
>> +			if (!pci_device_is_present(to_pci_dev(iommu->flush_target_dev)))
>> +				break;
> As a general approach, this is much better now.
>
> Please combine the nested if-clauses into one.
>
> Please amend the code comment with a spec reference, i.e.
> "(see Implementation Note in PCIe r6.1 sec 10.3.1)"
> so that readers of the code know where the magic number "1min+50%"
> is coming from.
>
> Is flush_target_dev guaranteed to always be a pci_dev?
>
> I'll let iommu maintainers comment on whether storing a flush_target_dev
> pointer is the right approach.  (May store a back pointer from
> struct intel_iommu to struct device_domain_info?)
>
> Maybe move the "to_pci_dev(iommu->flush_target_dev)" lookup outside the
> loop to avoid doing this over and over again?
>
> I think we still have a problem here if the device is not removed
> but simply takes a long time to respond to Invalidate Requests
> (as it is permitted to do per the Implementation Note).  We'll

If the hardware implenmentation didn't extend the PCIe spec, that

is possible and horrible case for current synchromous queue model

for ATS invalidation. but to wipe the concern and quote info not public

here, perhaps not proper for me.


Thanks,

Ethan

> busy-wait for the completion and potentially run into the watchdog's
> time limit again.  So I think you or someone else in your org should
> add OKRs to refactor the code so that it sleeps in-between polling
> for Invalidate Completions (instead of busy-waiting with interrupts
> disabled).
>
> Thanks,
>
> Lukas
>