linux-kernel - Re: [PATCH] iommu: Add Kconfig help text for IOMMU

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Message-ID: <CAHk-=wjmOAQnqJF-pW=fzMXb_Rk_J_Oi4ESBLmVPhxwBK4xfGg@mail.gmail.com>
Date:   Sun, 7 May 2023 11:52:18 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, iommu@...ts.linux.dev,
        Jason Gunthorpe <jgg@...dia.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>, Raj Ashok <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>
Subject: Re: [PATCH] iommu: Add Kconfig help text for IOMMU_SVA

On Sat, May 6, 2023 at 3:03 PM Jacob Pan <jacob.jun.pan@...ux.intel.com> wrote:
>
> Right, how about IOMMU_SHARING_CPU_PGTABLE?

I think from a VM / process angle, I'd actually prefer calling the
"pasid" part just that: IOMMU_PASID.

The VM code certainly understands about address space IDs, even if
people have called them different things: normal people called them
ASID's long ago, then Intel at some pointed decided that "PCID" made
sense as a name (narrator: "no it didn't"), and then you got that
combined "PASID" thing.

Now, it may be that this then goes hand-in-hand with other IOMMU code
that isn't *about* PASID itself, but that depends on PASID's being
present, and so I'd just expect IOMMU_PASID to be one of those options
that are selected by other options.

So maybe there is some part of IOMMU_SVA that is not about PASID
itself, but I really think that the PASID code itself should just have
that CONFIG_PASID around it.

End result: from a legibility standpoint, I think it could be as
simple as having that

    config IOMMU_SVA

option have a "select IOMMU_PASID".

Then make the VM/process PASID code depend on that. Maybe the "struct
device *" stuff makes more sense under CONFIG_IOMMU_SVA, ie things
like iopf_queue_add_device() and friends.

How does that sound? Maybe those two options then always end up going
together, but even if that is the case, I think from a VM/process
standpoint it makes a lot more sense to simply have a "PASID enabled"
option. It's much more understandable in that context, while something
like "IOMMU_SVA" really is just a random jumble of letters to a VM
person.

And while the individual words in IOMMU_SHARING_CPU_PGTABLE all make
sense, it's not clear what the combination means, and why it should
have anything to do with then having an address space identifier for
it.

                  Linus