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

From: Marc Zyngier <maz@kernel.org>
To: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
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@huawei.com>,
	<prime.zeng@hisilicon.com>, <linuxarm@openeuler.org>,
	Will Deacon <will@kernel.org>
Subject: Re: [RFC PATCH 3/5] KVM: ARM64: Add support for pinned VMIDs
Date: Tue, 09 Mar 2021 10:32:43 +0000	[thread overview]
Message-ID: <87tupky5v8.wl-maz@kernel.org> (raw)
In-Reply-To: <20210222155338.26132-4-shameerali.kolothum.thodi@huawei.com>

Hi Shameer,

[+Will]

On Mon, 22 Feb 2021 15:53:36 +0000,
Shameer Kolothum <shameerali.kolothum.thodi@huawei.com> 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.

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.

Thanks,

	M.

[1] https://patchwork.kernel.org/project/linux-arm-kernel/cover/20190724162534.7390-1-julien.grall@arm.com/

-- 
Without deviation from the norm, progress is not possible.

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