linux-kernel - Re: [PATCH 1/2] iommu/vt-d: Avoid unnecessary device TLB flush in map path

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Date: Tue, 9 Apr 2024 16:27:58 +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>, Joerg Roedel <joro@...tes.org>, "Will
 Deacon" <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] iommu/vt-d: Avoid unnecessary device TLB flush in map
 path

On 2024/4/7 22:42, Lu Baolu wrote:
> 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 the caching mode.
> This is inefficient and causes performance overhead.
> 
> Make device TLB invalidation behavior consistent between batched mode
> unmapping and strict mode unmapping. Device TLB invalidation should only
> be requested in the unmap path if the IOMMU is not in caching mode.
> 
> 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 | 6 +-----
>   1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 50eb9aed47cc..493b6a600394 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 (!cap_caching_mode(iommu->cap) && !map)
>   		iommu_flush_dev_iotlb(domain, addr, mask);
>   }
>   

The existing code works but kind of confusing. The iommu_flush_iotlb_psi()
helper will be called in both the map and unmap path. But device-TLB only
needed to be flushed in the unmap path since there is no chance for
device ATC to have cache for a non-present mapping. And only when caching
mode is reported, then should the helper be called in the map path. So the
fact is if caching mode is 0, the @map should always be false. If caching
mode is 1, then @map can be either false or true. To be simpler, it should
be enough to check @map before flushing device-TLB. no matter caching mode
or not.

-- 
Regards,
Yi Liu