Re: [PATCH 2/3] iommu/vt-d: Clear Present bit before tearing down PASID entry

[Dmytro Maluka <dmaluka@chromium.org>]

On Tue, Jan 13, 2026 at 11:00:47AM +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.
> 
> However, the IOMMU hardware may fetch these 64 bytes using multiple
> internal transactions (e.g., four 128-bit bursts). If a hardware fetch
> occurs simultaneously with the CPU zeroing the entry, the hardware could
> observe a "torn" entry — where some chunks are zeroed and others still
> contain old data — 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 a proper ownership handshake:
> 
> 1. Clear only the 'Present' (P) bit of the PASID entry. This tells the
>    hardware that the entry is no longer valid.
> 2. Execute the required invalidation sequence (PASID cache, IOTLB, and
>    Device-TLB flush) to ensure the hardware has released all cached
>    references to the entry.
> 3. Only after the flushes are complete, zero out the remaining fields of
>    the PASID entry.
> 
> Additionally, add an explicit clflush in intel_pasid_clear_entry() to
> ensure that the cleared entry is visible to the IOMMU on systems where
> memory coherency (ecap_coherent) is not supported.
> 
> Fixes: 0bbeb01a4faf ("iommu/vt-d: Manage scalalble mode PASID tables")
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> ---
>  drivers/iommu/intel/pasid.h | 12 ++++++++++++
>  drivers/iommu/intel/pasid.c |  9 +++++++--
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/intel/pasid.h b/drivers/iommu/intel/pasid.h
> index b4c85242dc79..35de1d77355f 100644
> --- a/drivers/iommu/intel/pasid.h
> +++ b/drivers/iommu/intel/pasid.h
> @@ -237,6 +237,18 @@ static inline void pasid_set_present(struct pasid_entry *pe)
>  	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);
> +}
> +
>  /*
>   * 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 298a39183996..4f36138448d8 100644
> --- a/drivers/iommu/intel/pasid.c
> +++ b/drivers/iommu/intel/pasid.c
> @@ -178,7 +178,8 @@ static struct pasid_entry *intel_pasid_get_entry(struct device *dev, u32 pasid)
>   * Interfaces for PASID table entry manipulation:
>   */
>  static void
> -intel_pasid_clear_entry(struct device *dev, u32 pasid, bool fault_ignore)
> +intel_pasid_clear_entry(struct intel_iommu *iommu, struct device *dev,
> +			u32 pasid, bool fault_ignore)
>  {
>  	struct pasid_entry *pe;
>  
> @@ -190,6 +191,9 @@ intel_pasid_clear_entry(struct device *dev, u32 pasid, bool fault_ignore)
>  		pasid_clear_entry_with_fpd(pe);
>  	else
>  		pasid_clear_entry(pe);
> +
> +	if (!ecap_coherent(iommu->ecap))
> +		clflush_cache_range(pe, sizeof(*pe));
>  }
>  
>  static void
> @@ -272,7 +276,7 @@ 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 +290,7 @@ 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(iommu, dev, pasid, fault_ignore);

Is it safe to do this with iommu->lock already unlocked?

>  	if (!fault_ignore)
>  		intel_iommu_drain_pasid_prq(dev, pasid);
>  }
> -- 
> 2.43.0
>