linux-kernel - Re: [PATCH 1/1] iommu/vt-d: Disallow dirty tracking if incoherent page walk

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Message-ID: <20250924120431.GF2617119@nvidia.com>
Date: Wed, 24 Sep 2025 09:04:31 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Lu Baolu <baolu.lu@...ux.intel.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Kevin Tian <kevin.tian@...el.com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] iommu/vt-d: Disallow dirty tracking if incoherent
 page walk

On Wed, Sep 24, 2025 at 04:34:46PM +0800, Lu Baolu wrote:
> Dirty page tracking relies on the IOMMU atomically updating the dirty bit
> in the paging-structure entry. For this operation to succeed, the paging-
> structure memory must be coherent between the IOMMU and the CPU. In
> another word, if the iommu page walk is incoherent, dirty page tracking
> doesn't work.
> 
> The Intel VT-d specification, Section 3.10 "Snoop Behavior" states:
> 
> "Remapping hardware encountering the need to atomically update A/EA/D bits
>  in a paging-structure entry that is not snooped will result in a non-
>  recoverable fault."
> 
> To prevent an IOMMU from being incorrectly configured for dirty page
> tracking when it is operating in an incoherent mode, mark SSADS as
> supported only when both ecap_slads and ecap_smpwc are supported.
> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>  drivers/iommu/intel/iommu.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Basically due to the above spec the IOMMU implementation would be
invalid to set slads and !smpwc ?

Jason