linux-kernel - Re: [PATCH 06/12] iommu/vt-d: Use cache_tag_flush_cm_range() in iotlb_sync

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Date: Sun, 7 Apr 2024 14:41:12 +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>
Cc: baolu.lu@...ux.intel.com, "Zhang, Tina" <tina.zhang@...el.com>,
 "Liu, Yi L" <yi.l.liu@...el.com>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/12] iommu/vt-d: Use cache_tag_flush_cm_range() in
 iotlb_sync_map

On 3/28/24 3:48 PM, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Monday, March 25, 2024 10:17 AM
>>
>> The iotlb_sync_map callback is called by the iommu core after non-present
>> to present mappings are created. The iommu driver uses this callback to
>> invalidate caches if IOMMU is working in caching mode and second-only
>> translation is used for the domain. Use cache_tag_flush_cm_range() in this
>> callback.
>>
>> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
>> ---
>>   drivers/iommu/intel/iommu.c | 22 +---------------------
>>   1 file changed, 1 insertion(+), 21 deletions(-)
>>
>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>> index 1c03d2dafb9d..2dcab1e5cd4d 100644
>> --- a/drivers/iommu/intel/iommu.c
>> +++ b/drivers/iommu/intel/iommu.c
>> @@ -1504,20 +1504,6 @@ static void iommu_flush_iotlb_psi(struct
>> intel_iommu *iommu,
>>   		iommu_flush_dev_iotlb(domain, addr, mask);
>>   }
>>
>> -/* Notification for newly created mappings */
>> -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);
>> -}
> 
> iommu_flush_write_buffer is for a quite different issue. it's clearer to
> keep it separated from the iotlb helpers.

The VT-d spec describes the write buffer flushing in section 6.8. It
states,

1. Write buffer flushing is required only for earlier VT-d hardware
    implementations. Newer hardware implementations are expected to NOT
    require this.
2. Write buffer flushing is related to cache invalidation. The hardware
    performs an implicit write-buffer-flushing as a pre-condition to
    the invalidation operations.

According to this, write buffer flushing is only required in the map
path when IOMMU is not in caching mode and the RWBF iommu capability is
set, which perfectly matches the case of cache_tag_flush_cm_range().

By doing it in cache_tag_flush_cm_range(), the code will be simpler and
easier for maintenance.

Best regards,
baolu