linux-kernel - RE: [PATCH v2 02/10] iommu: Introduce a new iommu_group_replace

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Message-ID: <BN9PR11MB52762F851C2712C2682015068CA29@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Tue, 14 Feb 2023 03:29:26 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Alex Williamson <alex.williamson@...hat.com>,
        Nicolin Chen <nicolinc@...dia.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "Raj, Ashok" <ashok.raj@...el.com>
Subject: RE: [PATCH v2 02/10] iommu: Introduce a new
 iommu_group_replace_domain() API

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Monday, February 13, 2023 10:46 PM
> 
> On Mon, Feb 13, 2023 at 02:24:40AM +0000, Tian, Kevin wrote:
> 
> > > This is because the cache tag and the io page table top are in
> > > different 64 bit words so atomic writes don't cover both, and thus the
> > > IOMMU HW could tear the two stores and mismatch the cache tag to the
> > > table top. This would corrupt the cache.
> >
> > VT-d spec recommends using 128bit cmpxchg instruction to update
> > page table pointer and DID together.
> 
> Oh really? Such a thing exists? That neat!
> 

yes. see cmpxchg_double().