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

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Message-ID: <CAKHBV27k8F0ZLy=RA=WhjJ7+C9JMHRRnKs=4W4pJMNmxrMEXxw@mail.gmail.com>
Date:   Mon, 21 Aug 2023 17:31:23 +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 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.