linux-kernel - Re: [RFC PATCH 2/4] iommu/vt-d: Add first level page table interfaces

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Message-ID: <20190925083516.GQ28074@xz-x1>
Date:   Wed, 25 Sep 2019 16:35:16 +0800
From:   Peter Xu <peterx@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Yi Sun <yi.y.sun@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Sun, Yi Y" <yi.y.sun@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC PATCH 2/4] iommu/vt-d: Add first level page table interfaces

On Wed, Sep 25, 2019 at 07:32:48AM +0000, Tian, Kevin wrote:
> > From: Lu Baolu [mailto:baolu.lu@...ux.intel.com]
> > Sent: Wednesday, September 25, 2019 2:52 PM
> > 
> > Hi Peter and Kevin,
> > 
> > On 9/25/19 1:24 PM, Peter Xu wrote:
> > > On Wed, Sep 25, 2019 at 04:38:31AM +0000, Tian, Kevin wrote:
> > >>> From: Peter Xu [mailto:peterx@...hat.com]
> > >>> Sent: Wednesday, September 25, 2019 12:31 PM
> > >>>
> > >>> On Tue, Sep 24, 2019 at 09:38:53AM +0800, Lu Baolu wrote:
> > >>>>>> intel_mmmap_range(domain, addr, end, phys_addr, prot)
> > >>>>>
> > >>>>> Maybe think of a different name..? mmmap seems a bit weird :-)
> > >>>>
> > >>>> Yes. I don't like it either. I've thought about it and haven't
> > >>>> figured out a satisfied one. Do you have any suggestions?
> > >>>
> > >>> How about at least split the word using "_"?  Like "mm_map", then
> > >>> apply it to all the "mmm*" prefixes.  Otherwise it'll be easily
> > >>> misread as mmap() which is totally irrelevant to this...
> > >>>
> > >>
> > >> what is the point of keeping 'mm' here? replace it with 'iommu'?
> > >
> > > I'm not sure of what Baolu thought, but to me "mm" makes sense itself
> > > to identify this from real IOMMU page tables (because IIUC these will
> > > be MMU page tables).  We can come up with better names, but IMHO
> > > "iommu" can be a bit misleading to let people refer to the 2nd level
> > > page table.
> > 
> > "mm" represents a CPU (first level) page table;
> > 
> > vs.
> > 
> > "io" represents an IOMMU (second level) page table.
> > 
> 
> IOMMU first level is not equivalent to CPU page table, though you can
> use the latter as the first level (e.g. in SVA). Especially here you are
> making IOVA->GPA as the first level, which is not CPU page table.
> 
> btw both levels are for "io" i.e. DMA purposes from VT-d p.o.v. They
> are just hierarchical structures implemented by VT-d, with slightly
> different format.

Regarding to the "slightly different format", do you mean the
extended-accessed bit?

Even if there are differences, they do look very similar.  If you see
the same chap 9.7 table, the elements are exactly called PTE, PDE,
PDPE, and so on - they're named exactly the same as MMU page tables.
With that, IMHO it still sounds reasonable if we want to relate this
"1st level iommu page table" with the existing MMU page table using
the "mm" prefix...

Regards,

-- 
Peter Xu