linux-kernel - Re: [PATCH 2/2] iommu/vt-d: Remove caching mode check before devtlb flush

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Message-ID: <04b9c7cb-e2ac-4dcf-8594-da101a98bc49@intel.com>
Date: Tue, 9 Apr 2024 16:36:45 +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 2/2] iommu/vt-d: Remove caching mode check before devtlb
 flush

On 2024/4/7 22:42, 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
> unrelated to the device TLB , therefore there is no need to check
> it before a device TLB invalidation operation.
> 
> Before the scalable mode is introduced, caching mode is treated as
> an indication that the driver is running in a VM guest. This is just
> a software contract as shadow page table is the only way to implement
> a virtual IOMMU. But the VT-d spec doesn't state this anywhere. After
> the scalable mode is introduced, this doesn't stand for anymore, as
> caching mode is not relevant for the first-stage translation. A virtual
> IOMMU implementation is free to support first-stage translation only
> with caching mode cleared.
> 
> Remove the caching mode check before device TLB invalidation to ensure
> compatibility with the scalable mode use cases.
> 
> Fixes: 792fb43ce2c9 ("iommu/vt-d: Enable Intel IOMMU scalable mode by default")
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>   drivers/iommu/intel/iommu.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 493b6a600394..681789b1258d 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -1501,7 +1501,7 @@ static void iommu_flush_iotlb_psi(struct intel_iommu *iommu,
>   	else
>   		__iommu_flush_iotlb_psi(iommu, did, pfn, pages, ih);
>   
> -	if (!cap_caching_mode(iommu->cap) && !map)
> +	if (!map)
>   		iommu_flush_dev_iotlb(domain, addr, mask);
>   }
>   

IMHO. this change can be merged with patch 01. And the reason is to make
the logic simple and clear.

> @@ -1575,8 +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)

you can also move the iommu_flush_dev_iotlb() call out of the iommu_array 
loop since it does not require checking iommu cap.

-- 
Regards,
Yi Liu