linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Date:   Tue, 8 Jun 2021 10:49:56 +1000
From:   David Gibson <david@...son.dropbear.id.au>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        David Woodhouse <dwmw2@...radead.org>,
        Jason Wang <jasowang@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Thu, Jun 03, 2021 at 06:49:20AM +0000, Tian, Kevin wrote:
> > From: David Gibson
> > Sent: Thursday, June 3, 2021 1:09 PM
> [...]
> > > > In this way the SW mode is the same as a HW mode with an infinite
> > > > cache.
> > > >
> > > > The collaposed shadow page table is really just a cache.
> > > >
> > >
> > > OK. One additional thing is that we may need a 'caching_mode"
> > > thing reported by /dev/ioasid, indicating whether invalidation is
> > > required when changing non-present to present. For hardware
> > > nesting it's not reported as the hardware IOMMU will walk the
> > > guest page table in cases of iotlb miss. For software nesting
> > > caching_mode is reported so the user must issue invalidation
> > > upon any change in guest page table so the kernel can update
> > > the shadow page table timely.
> > 
> > For the fist cut, I'd have the API assume that invalidates are
> > *always* required.  Some bypass to avoid them in cases where they're
> > not needed can be an additional extension.
> > 
> 
> Isn't a typical TLB semantics is that non-present entries are not
> cached thus invalidation is not required when making non-present
> to present?

Usually, but not necessarily.

> It's true to both CPU TLB and IOMMU TLB.

I don't think it's entirely true of the CPU TLB on all ppc MMU models
(of which there are far too many).

> In reality
> I feel there are more usages built on hardware nesting than software
> nesting thus making default following hardware TLB behavior makes
> more sense...

I'm arguing for always-require-invalidate because it's strictly more
general.  Requiring the invalidate will support models that don't
require it in all cases; we just make the invalidate a no-op.  The
reverse is not true, so we should tackle the general case first, then
optimize.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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