linux-kernel - Re: [PATCH v2 1/1] iommu/vt-d: Remove caching mode check before device TLB flush

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Message-ID: <7e78917f-f84c-4e98-a612-73b8013ae367@intel.com>
Date: Wed, 10 Apr 2024 14:30:20 +0800
From: Yi Liu <yi.l.liu@...el.com>
To: Lu Baolu <baolu.lu@...ux.intel.com>, <iommu@...ts.linux.dev>
CC: Kevin Tian <kevin.tian@...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 v2 1/1] iommu/vt-d: Remove caching mode check before
 device TLB flush

On 2024/4/10 13:58, 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 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.
> 
> iommu_flush_iotlb_psi() is called in map and unmap paths. The caching
> mode check before device TLB invalidation will cause device TLB
> invalidation always issued if IOMMU is not running in caching mode.
> This is wrong and causes unnecessary performance overhead.

I don't think the original code is wrong. As I replied before, if CM==0,
the iommu_flush_iotlb_psi() is only called in unmap path, in which the
@map is false. [1] The reason to make the change is to make the logic
simpler. :)

[1] 
https://lore.kernel.org/linux-iommu/ac7fdf0f-ecb9-415f-9c1b-600536b92ca5@intel.com/

> The removal of caching mode check in intel_flush_iotlb_all() doesn't
> impact anything no matter the IOMMU is working in caching mode or not.
> Commit <29b32839725f> ("iommu/vt-d: Do not use flush-queue when
> caching-mode is on") has already disabled flush-queue for caching mode,
> hence caching mode will never call intel_flush_iotlb_all().
> 
> Fixes: bf92df30df90 ("intel-iommu: Only avoid flushing device IOTLB for domain ID 0 in caching mode")
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>   drivers/iommu/intel/iommu.c | 9 ++-------
>   1 file changed, 2 insertions(+), 7 deletions(-)
> 
> Change log:
> 
> v2:
>   - 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 50eb9aed47cc..681789b1258d 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);
>   }
>   
> @@ -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);

why not one more step to move it out of the iommu_array loop? There is
no more reason to keep it in the loop.

>   	}
>   
>   	if (dmar_domain->nested_parent)

-- 
Regards,
Yi Liu