linux-kernel - Re: [PATCH v2 1/3] iommu/vt-d: Clear Present bit before tearing down PASID entry

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Message-ID: <aW-JpXkqEpGTL7eq@google.com>
Date: Tue, 20 Jan 2026 14:56:53 +0100
From: Dmytro Maluka <dmaluka@...omium.org>
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>, Jason Gunthorpe <jgg@...dia.com>,
	Samiullah Khawaja <skhawaja@...gle.com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	"Vineeth Pillai (Google)" <vineeth@...byteword.org>,
	Aashish Sharma <aashish@...hishsharma.net>
Subject: Re: [PATCH v2 1/3] iommu/vt-d: Clear Present bit before tearing down
 PASID entry

On Tue, Jan 20, 2026 at 02:18:12PM +0800, Lu Baolu wrote:
> The Intel VT-d Scalable Mode PASID table entry consists of 512 bits (64
> bytes). When tearing down an entry, the current implementation zeros the
> entire 64-byte structure immediately using multiple 64-bit writes.
> 
> Since the IOMMU hardware may fetch these 64 bytes using multiple
> internal transactions (e.g., four 128-bit bursts), updating or zeroing
> the entire entry while it is active (P=1) risks a "torn" read. If a
> hardware fetch occurs simultaneously with the CPU zeroing the entry, the
> hardware could observe an inconsistent state, leading to unpredictable
> behavior or spurious faults.
> 
> Follow the "Guidance to Software for Invalidations" in the VT-d spec
> (Section 6.5.3.3) by implementing the recommended ownership handshake:
> 
> 1. Clear only the 'Present' (P) bit of the PASID entry.
> 2. Use a dma_wmb() to ensure the cleared bit is visible to hardware
>    before proceeding.
> 3. Execute the required invalidation sequence (PASID cache, IOTLB, and
>    Device-TLB flush) to ensure the hardware has released all cached
>    references.
> 4. Only after the flushes are complete, zero out the remaining fields
>    of the PASID entry.
> 
> Also, add a dma_wmb() in pasid_set_present() to ensure that all other
> fields of the PASID entry are visible to the hardware before the Present
> bit is set.
> 
> Fixes: 0bbeb01a4faf ("iommu/vt-d: Manage scalalble mode PASID tables")
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>

Reviewed-by: Dmytro Maluka <dmaluka@...omium.org>

> ---
>  drivers/iommu/intel/pasid.h | 14 ++++++++++++++
>  drivers/iommu/intel/pasid.c |  6 +++++-
>  2 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/intel/pasid.h b/drivers/iommu/intel/pasid.h
> index b4c85242dc79..0b303bd0b0c1 100644
> --- a/drivers/iommu/intel/pasid.h
> +++ b/drivers/iommu/intel/pasid.h
> @@ -234,9 +234,23 @@ static inline void pasid_set_wpe(struct pasid_entry *pe)
>   */
>  static inline void pasid_set_present(struct pasid_entry *pe)
>  {
> +	dma_wmb();
>  	pasid_set_bits(&pe->val[0], 1 << 0, 1);
>  }
>  
> +/*
> + * Clear the Present (P) bit (bit 0) of a scalable-mode PASID table entry.
> + * This initiates the transition of the entry's ownership from hardware
> + * to software. The caller is responsible for fulfilling the invalidation
> + * handshake recommended by the VT-d spec, Section 6.5.3.3 (Guidance to
> + * Software for Invalidations).
> + */
> +static inline void pasid_clear_present(struct pasid_entry *pe)
> +{
> +	pasid_set_bits(&pe->val[0], 1 << 0, 0);
> +	dma_wmb();
> +}
> +
>  /*
>   * Setup Page Walk Snoop bit (Bit 87) of a scalable mode PASID
>   * entry.
> diff --git a/drivers/iommu/intel/pasid.c b/drivers/iommu/intel/pasid.c
> index 3e2255057079..eb069aefa4fa 100644
> --- a/drivers/iommu/intel/pasid.c
> +++ b/drivers/iommu/intel/pasid.c
> @@ -272,7 +272,7 @@ void intel_pasid_tear_down_entry(struct intel_iommu *iommu, struct device *dev,
>  
>  	did = pasid_get_domain_id(pte);
>  	pgtt = pasid_pte_get_pgtt(pte);
> -	intel_pasid_clear_entry(dev, pasid, fault_ignore);
> +	pasid_clear_present(pte);
>  	spin_unlock(&iommu->lock);
>  
>  	if (!ecap_coherent(iommu->ecap))
> @@ -286,6 +286,10 @@ void intel_pasid_tear_down_entry(struct intel_iommu *iommu, struct device *dev,
>  		iommu->flush.flush_iotlb(iommu, did, 0, 0, DMA_TLB_DSI_FLUSH);
>  
>  	devtlb_invalidation_with_pasid(iommu, dev, pasid);
> +	intel_pasid_clear_entry(dev, pasid, fault_ignore);
> +	if (!ecap_coherent(iommu->ecap))
> +		clflush_cache_range(pte, sizeof(*pte));
> +
>  	if (!fault_ignore)
>  		intel_iommu_drain_pasid_prq(dev, pasid);
>  }
> -- 
> 2.43.0