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 08:53:34 +0800
From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
To: Lu Baolu <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 4/15/2024 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() ?


Thanks,
Ethan

>   
> @@ -1579,8 +1575,7 @@ static void intel_flush_iotlb_all(struct iommu_domain *domain)
>   			iommu->flush.flush_iotlb(iommu, did, 0, 0,
>   						 DMA_TLB_DSI_FLUSH);
>   
> -		if (!cap_caching_mode(iommu->cap))
> -			iommu_flush_dev_iotlb(dmar_domain, 0, MAX_AGAW_PFN_WIDTH);
> +		iommu_flush_dev_iotlb(dmar_domain, 0, MAX_AGAW_PFN_WIDTH);
>   	}
>   
>   	if (dmar_domain->nested_parent)