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

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Date: Mon, 8 Apr 2024 14:03:29 -0700
From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
To: Lu Baolu <baolu.lu@...ux.intel.com>
Cc: iommu@...ts.linux.dev, Kevin Tian <kevin.tian@...el.com>, Yi Liu
 <yi.l.liu@...el.com>, Joerg Roedel <joro@...tes.org>, Will Deacon
 <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 linux-kernel@...r.kernel.org, jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH 2/2] iommu/vt-d: Remove caching mode check before devtlb
 flush

Hi Lu,

On Sun,  7 Apr 2024 22:42:32 +0800, Lu Baolu <baolu.lu@...ux.intel.com>
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.
> 
I agree with the changes below, what about this CM check:

/* 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);

We are still tying devTLB flush to CM=1, no?

If we are running in the guest with second level page table (shadowed), can
we decide if devTLB flush is needed based on ATS enable just as the rest of
the 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);
>  }
>  
> @@ -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)


Thanks,

Jacob