Hi Marc,
> -Original Message-
> From: Marc Zyngier [mailto:m...@kernel.org]
> Sent: 09 March 2021 10:33
> To: Shameerali Kolothum Thodi
> Cc: linux-arm-ker...@lists.infradead.org; io...@lists.linux-foundation.org;
> kvmarm@lists.cs.columbia.edu; alex.william...@redhat.com;
> jean-phili...@linaro.org; eric.au...@redhat.com; zhangfei@linaro.org;
> Jonathan Cameron ; Zengtao (B)
> ; linux...@openeuler.org; Will Deacon
>
> Subject: Re: [RFC PATCH 3/5] KVM: ARM64: Add support for pinned VMIDs
>
> Hi Shameer,
>
> [+Will]
>
> On Mon, 22 Feb 2021 15:53:36 +,
> Shameer Kolothum wrote:
> >
> > On an ARM64 system with a SMMUv3 implementation that fully supports
> > Broadcast TLB Maintenance(BTM) feature, the CPU TLB invalidate
> > instructions are received by SMMU. This is very useful when the
> > SMMU shares the page tables with the CPU(eg: Guest SVA use case).
> > For this to work, the SMMU must use the same VMID that is allocated
> > by KVM to configure the stage 2 translations.
> >
> > At present KVM VMID allocations are recycled on rollover and may
> > change as a result. This will create issues if we have to share
> > the KVM VMID with SMMU. Hence, we spilt the KVM VMID space into
> > two, the first half follows the normal recycle on rollover policy
> > while the second half of the VMID pace is used to allocate pinned
> > VMIDs. This feature is enabled based on a command line option
> > "kvm-arm.pinned_vmid_enable".
>
> I think this is the wrong approach. Instead of shoving the notion of
> pinned VMID into the current allocator, which really isn't designed
> for this, it'd be a lot better if we aligned the KVM VMID allocator
> with the ASID allocator, which already has support for pinning and is
> in general much more efficient.
Ok. Agree that this is not efficient, but was easy to prototype something :)
> Julien Grall worked on such a series[1] a long while ago, which got
> stalled because of the 32bit KVM port. Since we don't have this burden
> anymore, I'd rather you look in that direction instead of wasting half
> of the VMID space on potentially pinned VMIDs.
Sure. I will check that and work on it.
Thanks,
Shameer
> Thanks,
>
> M.
>
> [1]
> https://patchwork.kernel.org/project/linux-arm-kernel/cover/20190724162534
> .7390-1-julien.gr...@arm.com/
>
>
> --
> Without deviation from the norm, progress is not possible.
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