Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230506133134.1492395-1-jacob.jun.pan@linux.intel.com>
 <CAHk-=wgUiAtiszwseM1p2fCJ+sC4XWQ+YN4TanFhUgvUqjr9Xw@mail.gmail.com> <20230506150741.2e3d2dcc@jacob-builder>
In-Reply-To: <20230506150741.2e3d2dcc@jacob-builder>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 7 May 2023 11:52:18 -0700
Message-ID: <CAHk-=wjmOAQnqJF-pW=fzMXb_Rk_J_Oi4ESBLmVPhxwBK4xfGg@mail.gmail.com>
Subject: Re: [PATCH] iommu: Add Kconfig help text for IOMMU_SVA
To:     Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, iommu@lists.linux.dev,
        Jason Gunthorpe <jgg@nvidia.com>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Will Deacon <will@kernel.org>, Raj Ashok <ashok.raj@intel.com>,
        "Tian, Kevin" <kevin.tian@intel.com>, Yi Liu <yi.l.liu@intel.com>,
        "Yu, Fenghua" <fenghua.yu@intel.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Sat, May 6, 2023 at 3:03=E2=80=AFPM Jacob Pan <jacob.jun.pan@linux.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