linux-kernel - Re: [PATCH v3 1/1] iommu/vt-d: Remove caching mode check before device TLB flush

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Date: Tue, 16 Apr 2024 14:30:38 +0800
From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, iommu@...ts.linux.dev
Cc: Kevin Tian <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>,
 Jacob Pan <jacob.jun.pan@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] iommu/vt-d: Remove caching mode check before
 device TLB flush

On 4/16/2024 10:57 AM, Baolu Lu wrote:
> On 4/16/24 8:53 AM, Ethan Zhao wrote:
>> On 4/15/2024 9:38 AM, Lu Baolu wrote:
>>> The Caching Mode (CM) of the Intel IOMMU indicates if the hardware
>>> implementation caches not-present or erroneous translation-structure
>>> entries except for the first-stage translation. The caching mode is
>>> irrelevant to the device TLB, therefore there is no need to check it
>>> before a device TLB invalidation operation.
>>>
>>> Remove two caching mode checks before device TLB invalidation in the
>>> driver. The removal of these checks doesn't change the driver's 
>>> behavior
>>> in critical map/unmap paths. Hence, there is no functionality or
>>> performance impact, especially since commit <29b32839725f> 
>>> ("iommu/vt-d:
>>> Do not use flush-queue when caching-mode is on") has already disabled
>>> flush-queue for caching mode. Therefore, caching mode will never call
>>> intel_flush_iotlb_all().
>>>
>>> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
>>> Reviewed-by: Kevin Tian <kevin.tian@...el.com>
>>> ---
>>>   drivers/iommu/intel/iommu.c | 9 ++-------
>>>   1 file changed, 2 insertions(+), 7 deletions(-)
>>>
>>> Change log:
>>> v3:
>>>   - It turned out that the removals don't change the driver's behavior,
>>>     hence change it from a fix patch to a cleanup one.
>>>   - No functionality changes.
>>> v2: 
>>> https://lore.kernel.org/lkml/20240410055823.264501-1-baolu.lu@linux.intel.com/
>>>   - Squash two patches into a single one.
>>>   - No functionality changes.
>>> v1: 
>>> https://lore.kernel.org/linux-iommu/20240407144232.190355-1-baolu.lu@linux.intel.com/
>>>
>>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>>> index a7ecd90303dc..f0a67e9d9faf 100644
>>> --- a/drivers/iommu/intel/iommu.c
>>> +++ b/drivers/iommu/intel/iommu.c
>>> @@ -1501,11 +1501,7 @@ static void iommu_flush_iotlb_psi(struct 
>>> intel_iommu *iommu,
>>>       else
>>>           __iommu_flush_iotlb_psi(iommu, did, pfn, pages, ih);
>>> -    /*
>>> -     * In caching mode, changes of pages from non-present to 
>>> present require
>>> -     * flush. However, device IOTLB doesn't need to be flushed in 
>>> this case.
>>> -     */
>>> -    if (!cap_caching_mode(iommu->cap) || !map)
>>> +    if (!map)
>>>           iommu_flush_dev_iotlb(domain, addr, mask);
>>>   }
>>
>> Given devTLB flushing is irrelavent to CM, put iommu_flush_dev_iotlb()
>> in iommu_flush_iotlb_psi() and called with CM checking context is not
>> reasonable. the logic is buggy.
>>
>> static void __mapping_notify_one(struct intel_iommu *iommu, struct 
>> dmar_domain *domain,
>>                   unsigned long pfn, unsigned int pages)
>> {
>>      /*
>>       * It's a non-present to present mapping. Only flush if caching 
>> mode
>>       * and second level.
>>       */
>>      if (cap_caching_mode(iommu->cap) && !domain->use_first_level)
>>          iommu_flush_iotlb_psi(iommu, domain, pfn, pages, 0, 1);
>>      else
>>          iommu_flush_write_buffer(iommu);
>>
>>
>> then how about fold all CM checking logic in iommu_flush_iotlb_psi()
>> or speperate iommu_flush_dev_iotlb() from iommu_flush_iotlb_psi() ?
>
> I am refactoring the code with a new series.
>
> https://lore.kernel.org/linux-iommu/20240410020844.253535-1-baolu.lu@linux.intel.com/ 
>

Great, thx.

>
> Best regards,
> baolu