linux-kernel - Re: [PATCH v2 2/2] iommu/vt-d: Fix NULL domain on device release

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Message-ID: <4fadb386-e441-4464-9af1-0d369aed717e@linux.intel.com>
Date: Mon, 4 Mar 2024 17:39:03 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: "Tian, Kevin" <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Jason Gunthorpe <jgg@...pe.ca>, "Badger, Eric" <ebadger@...estorage.com>
Cc: baolu.lu@...ux.intel.com, "iommu@...ts.linux.dev"
 <iommu@...ts.linux.dev>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] iommu/vt-d: Fix NULL domain on device release

On 2024/3/4 16:59, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>> Sent: Monday, March 4, 2024 4:07 PM
>>
>> On 2024/3/4 15:36, Tian, Kevin wrote:
>>>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>>>> Sent: Thursday, February 29, 2024 5:46 PM
>>>>
>>>> +
>>>> +/*
>>>> + * Cache invalidation for changes to a scalable-mode context table
>>>> + * entry.
>>>> + *
>>>> + * Section 6.5.3.3 of the VT-d spec:
>>>> + * - Device-selective context-cache invalidation;
>>>> + * - Domain-selective PASID-cache invalidation to affected domains
>>>> + *   (can be skipped if all PASID entries were not-present);
>>>> + * - Domain-selective IOTLB invalidation to affected domains;
>>>
>>> the spec talks about domain-selective but the code actually does
>>> global invalidation.
>>
>> I should have included the following comments below:
>>
>> /* Given that we have no idea about which domain IDs and PASIDs were
>>    * used in the pasid table, upgrade them to global PASID and IOTLB
>>    * cache invalidation. This doesn't impact the performance significantly
>>    * as the clearing context entry is not a critical path.
>>    */
>>
> 
> but then it affects all other perf-critical paths which rely on the cache
> for other devices...

You are right. Good consideration.

> 
> It's preferable to restrict overhead to this release path only e.g. walking
> the PASID table to identify affected DIDs and PASIDs instead of expanding
> the impact to system wide.

The sm_context_flush_caches() could be used in two different paths:
- Deferred attachment case;
- Normal device release path.

For the formal case, we have to use global cache invalidation; but the
the latter case, it's fine to skip these cache invalidation. The new
helper probably looks like below.

/*
  * Cache invalidation for changes to a scalable-mode context table
  * entry.
  *
  * Section 6.5.3.3 of the VT-d spec:
  * - Device-selective context-cache invalidation;
  * - Domain-selective PASID-cache invalidation to affected domains
  *   (can be skipped if all PASID entries were not-present);
  * - Domain-selective IOTLB invalidation to affected domains;
  * - Global Device-TLB invalidation to affected functions.
  *
  * For kdump cases, old valid entries may be cached due to the in-flight
  * DMA and copied pgtable, but there is no unmapping behaviour for them,
  * thus we need explicit cache flushes for all affected domain IDs and
  * PASIDs used in the copied PASID table. Given that we have no idea about
  * which domain IDs and PASIDs were used in the copied tables, upgrade
  * them to global PASID and IOTLB cache invalidation.
  *
  * For normal case, the iommu has been parked in blocking state. All PASID
  * entries are in non-present now. Skip PASID and IOTLB cache invalidation.
  */
static void sm_context_flush_caches(struct device *dev)
{
         struct device_domain_info *info = dev_iommu_priv_get(dev);
         struct intel_iommu *iommu = info->iommu;

         iommu->flush.flush_context(iommu, 0, PCI_DEVID(info->bus, 
info->devfn),
                                    DMA_CCMD_MASK_NOBIT, 
DMA_CCMD_DEVICE_INVL);
         if (context_copied(iommu, info->bus, info->devfn)) {
                 qi_flush_pasid_cache(iommu, 0, QI_PC_GLOBAL, 0);
                 iommu->flush.flush_iotlb(iommu, 0, 0, 0, 
DMA_TLB_GLOBAL_FLUSH);
         }
         devtlb_invalidation_with_pasid(iommu, dev, IOMMU_NO_PASID);
}

Does it look good for you?

Best regards,
baolu