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

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Message-ID: <BN9PR11MB52765D0E2B4A39CA857900ED8C8FA@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Wed, 14 Jan 2026 07:32:22 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>, "Will
 Deacon" <will@...nel.org>, Robin Murphy <robin.murphy@....com>, "Jason
 Gunthorpe" <jgg@...dia.com>
CC: Dmytro Maluka <dmaluka@...omium.org>, Samiullah Khawaja
	<skhawaja@...gle.com>, "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/3] iommu/vt-d: Clear Present bit before tearing down
 PASID entry

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Tuesday, January 13, 2026 11:01 AM
> 
> 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    hardware that the entry is no longer valid.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.
> 

better split the clflush part into a separate patch