RE: [RFC PATCH 3/5] KVM: ARM64: Add support for pinned VMIDs

Hi Marc,

-----Original Message-----
From: Marc Zyngier [mailto:maz@kernel.org]
Sent: 09 March 2021 10:33
To: Shameerali Kolothum Thodi <redacted>
Cc: linux-arm-kernel@lists.infradead.org; iommu@lists.linux-foundation.org;
kvmarm@lists.cs.columbia.edu; alex.williamson@redhat.com;
jean-philippe@linaro.org; eric.auger@redhat.com; zhangfei.gao@linaro.org;
Jonathan Cameron [off-list ref]; Zengtao (B)
[off-list ref]; linuxarm@openeuler.org; Will Deacon
[off-list ref]
Subject: Re: [RFC PATCH 3/5] KVM: ARM64: Add support for pinned VMIDs

Hi Shameer,

[+Will]

On Mon, 22 Feb 2021 15:53:36 +0000,
Shameer Kolothum [off-list ref] wrote:

quoted

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.grall@arm.com/


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