linux-kernel - RE: [PATCH 2/2] iommu/vt-d: Remove caching mode check before devtlb flush

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Date: Fri, 12 Apr 2024 03:13:33 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC: Baolu Lu <baolu.lu@...ux.intel.com>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "Liu, Yi L" <yi.l.liu@...el.com>, Joerg Roedel
	<joro@...tes.org>, Will Deacon <will@...nel.org>, Robin Murphy
	<robin.murphy@....com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/2] iommu/vt-d: Remove caching mode check before devtlb
 flush

> From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> Sent: Friday, April 12, 2024 12:18 AM
> 
> Hi Kevin,
> 
> On Wed, 10 Apr 2024 23:23:57 +0000, "Tian, Kevin" <kevin.tian@...el.com>
> wrote:
> 
> > > From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > > Sent: Thursday, April 11, 2024 12:20 AM
> > >
> > > Hi Kevin,
> > >
> > > On Wed, 10 Apr 2024 00:32:06 +0000, "Tian, Kevin"
> <kevin.tian@...el.com>
> > > wrote:
> > >
> > > > > From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > > > > Sent: Wednesday, April 10, 2024 1:32 AM
> > > > >
> > > > > If the guest uses SL page tables in vIOMMU, we don;t expose ATS to
> > > > > the guest. So ATS is not relevant here, does't matter map or unmap.
> > > > >
> > > >
> > > > ATS is orthogonal to SL vs. FL. Where is this restriction coming
> > > > from?
> > > For practical purposes, what would be the usage to have SL in the guest
> > > and ATS enabled. i.e. shadowing SL but directly expose ATS?
> > >
> >
> > ATS is about the protocol between device and iommu to look up
> > translations. Why does it care about internal paging layout in
> > iommu?
> >
> Maybe the original intent was missed, I was suggesting the devTLB flush
> should be based on ATS cap (as you said here) not map/unmap.
> 
> -       /*
> -        * In caching mode, changes of pages from non-present to present
> require
> -        * flush. However, device IOTLB doesn't need to be flushed in this case.
> -        */
> -       if (!cap_caching_mode(iommu->cap) || !map)
> -               iommu_flush_dev_iotlb(domain, addr, mask);
> +       iommu_flush_dev_iotlb(domain, addr, mask);
> 

We need check both, as devtlb doesn't cache non-present 
so the invalidation is required only for unmap.

Here just the check of caching mode is irrelevant.