linux-kernel - Re: [PATCH 2/5] iommu/vt-d: Set SNP bit only in second-level page table entries

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Message-ID: <20220504133142.GE49344@nvidia.com>
Date:   Wed, 4 May 2022 10:31:42 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] iommu/vt-d: Set SNP bit only in second-level page
 table entries

On Wed, May 04, 2022 at 03:25:50PM +0800, Baolu Lu wrote:
> Hi Jason,
> 
> On 2022/5/2 21:05, Jason Gunthorpe wrote:
> > On Sun, May 01, 2022 at 07:24:31PM +0800, Lu Baolu wrote:
> > > The SNP bit is only valid for second-level PTEs. Setting this bit in the
> > > first-level PTEs has no functional impact because the Intel IOMMU always
> > > ignores the same bit in first-level PTEs. Anyway, let's check the page
> > > table type before setting SNP bit in PTEs to make the code more readable.
> > Shouldn't this be tested before setting force_snooping and not during
> > every map?
> 
> The check is in the following patch. This just makes sure that SNP is
> only set in second-level page table entries.

I think you should add a 2nd flag to indicate 'set SNP bit in PTEs'
and take care of computing that flag in the enforce_no_snoop function

Jason