linux-kernel - RE: [PATCH v4 15/32] vfio: introduce KVM-owned IOMMU type

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Date:   Sat, 19 Mar 2022 07:47:22 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "schnelle@...ux.ibm.com" <schnelle@...ux.ibm.com>,
        "farman@...ux.ibm.com" <farman@...ux.ibm.com>,
        "pmorel@...ux.ibm.com" <pmorel@...ux.ibm.com>,
        "borntraeger@...ux.ibm.com" <borntraeger@...ux.ibm.com>,
        "hca@...ux.ibm.com" <hca@...ux.ibm.com>,
        "gor@...ux.ibm.com" <gor@...ux.ibm.com>,
        "gerald.schaefer@...ux.ibm.com" <gerald.schaefer@...ux.ibm.com>,
        "agordeev@...ux.ibm.com" <agordeev@...ux.ibm.com>,
        "svens@...ux.ibm.com" <svens@...ux.ibm.com>,
        "frankja@...ux.ibm.com" <frankja@...ux.ibm.com>,
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        "imbrenda@...ux.ibm.com" <imbrenda@...ux.ibm.com>,
        "vneethv@...ux.ibm.com" <vneethv@...ux.ibm.com>,
        "oberpar@...ux.ibm.com" <oberpar@...ux.ibm.com>,
        "freude@...ux.ibm.com" <freude@...ux.ibm.com>,
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        "will@...nel.org" <will@...nel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: RE: [PATCH v4 15/32] vfio: introduce KVM-owned IOMMU type

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Friday, March 18, 2022 9:46 PM
> 
> On Fri, Mar 18, 2022 at 07:01:19AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Tuesday, March 15, 2022 10:55 PM
> > >
> > > The first level iommu_domain has the 'type1' map and unmap and pins
> > > the pages. This is the 1:1 map with the GPA and ends up pinning all
> > > guest memory because the point is you don't want to take a memory pin
> > > on your performance path
> > >
> > > The second level iommu_domain points to a single IO page table in GPA
> > > and is created/destroyed whenever the guest traps to the hypervisor to
> > > manipulate the anchor (ie the GPA of the guest IO page table).
> > >
> >
> > Can we use consistent terms as used in iommufd and hardware, i.e.
> > with first-level/stage-1 referring to the child (GIOVA->GPA) which is
> > further nested on second-level/stage-2 as the parent (GPA->HPA)?
> 
> Honestly I don't like injecting terms that only make sense for
> virtualization into iommu/vfio land.

1st/2nd-level or stage-1/2 are hardware terms not tied to virtualization.
GIOVA/GPA are just examples in this story.

> 
> That area is intended to be general. If you use what it exposes for
> virtualization, then great.
> 
> This is why I prefer to use 'user page table' when talking about the
> GIOVA->GPA or Stage 1 map because it is a phrase independent of
> virtualization or HW and clearly conveys what it is to the kernel and
> its inherent order in the translation scheme.

I fully agree with this point. The confusion only comes when you
start talking about first/second level in a way incompatible with
what iommu/vfio guys typically understand. 😊

> 
> The S1/S2 is gets confusing because the HW people choose those names
> so that S1 is the first translation a TLP sees and S2 is the second.
> 
> But from a software model, the S2 is the first domain created and the
> first level of the translation tree, while the S1 is the second domain
> created and the second level of the translation tree. ie the natural
> SW numbers are backwards.

Yes, I got this point.

> 
> And I know Matthew isn't working on HW that has the S1/S2 HW naming :)
> 
> But yes, should try harder to have good names. Maybe it will be
> clearer with code.
> 

Yes. let's try to use 'user page table' as possible. But if the levels/stages
are inevitable in description I prefer to staying with the hardware terms
given iommu driver has to deal with hardware naming things.

Thanks
Kevin