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

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Message-ID: <da521760-c2d7-432e-8ace-8793d520ab09@linux.intel.com>
Date: Tue, 16 Apr 2024 10:57:08 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Ethan Zhao <haifeng.zhao@...ux.intel.com>, iommu@...ts.linux.dev
Cc: baolu.lu@...ux.intel.com, 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 4/16/24 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/

Best regards,
baolu