linux-kernel - Re: [RFC PATCH v1 3/8] iommu/arm-smmu-v3-sva: Allocate new ASID from installed

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Date:   Mon, 21 Aug 2023 21:38:40 +0800
From:   Michael Shavit <mshavit@...gle.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, will@...nel.org, nicolinc@...dia.com,
        tina.zhang@...el.com, jean-philippe@...aro.org,
        robin.murphy@....com
Subject: Re: [RFC PATCH v1 3/8] iommu/arm-smmu-v3-sva: Allocate new ASID from installed_smmus

On Mon, Aug 21, 2023 at 7:54 PM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Mon, Aug 21, 2023 at 05:31:23PM +0800, Michael Shavit wrote:
> > On Fri, Aug 18, 2023 at 2:38 AM Jason Gunthorpe <jgg@...dia.com> wrote:
> > >
> > > On Fri, Aug 18, 2023 at 02:16:25AM +0800, Michael Shavit wrote:
> > > > Pick an ASID that is within the supported range of all SMMUs that the
> > > > domain is installed to.
> > > >
> > > > Signed-off-by: Michael Shavit <mshavit@...gle.com>
> > > > ---
> > >
> > > This seems like a pretty niche scenario, maybe we should just keep a
> > > global for the max ASID?
> > >
> > > Otherwise we need a code to change the ASID, even for non-SVA domains,
> > > when the domain is installed in different devices if the current ASID
> > > is over the instance max..
> >
> > This RFC took the other easy way out for this problem by rejecting
> > attaching a domain if its currently assigned ASID/VMID
> > is out of range when attaching to a new SMMU. But I'm not sure
> > which of the two options is the right trade-off.
> > Especially if we move VMID to a global allocator (which I plan to add
> > for v2), setting a global maximum for VMID of 256 sounds small.
>
> IMHO the simplest and best thing is to make both vmid and asid as
> local allocators. Then alot of these problems disappear

Well that does sound like the most flexible, but IMO quite a lot more
complicated.

I'll post a v2 RFC that removes the `iommu/arm-smmu-v3: Add list of
installed_smmus` patch and uses a flat master list in smmu_domain as
suggested by Robin, for comparison with the v1. But at a glance using a
local allocator would require:
1. Keeping that patch so we can track the asid/vmid for a domain on a
per smmu instance
2. Keeping a map in the smmu struct so that arm_smmu_share_asid can
find any arm_smmu_installed_smmu that need to have their asid updated
(on a loop over every smmu the domain in arm_smmu_mmu_notifier_get is
attached to, which just at a glance looks headache inducing because of
sva's piggybacking on the rid domain.)