linux-kernel - Re: [PATCH v4 00/16] kvm/arm: Align the VMID allocation with the arm64 ASID one

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Date:   Thu, 22 Apr 2021 17:08:48 +0100
From:   Will Deacon <will@...nel.org>
To:     Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, maz@...nel.org,
        catalin.marinas@....com, james.morse@....com,
        julien.thierry.kdev@...il.com, suzuki.poulose@....com,
        jean-philippe@...aro.org, julien@....org, linuxarm@...wei.com
Subject: Re: [PATCH v4 00/16] kvm/arm: Align the VMID allocation with the
 arm64 ASID one

On Wed, Apr 14, 2021 at 12:22:56PM +0100, Shameer Kolothum wrote:
> Hi,
> 
> This is an attempt to revive this series originally posted by
> Julien Grall[1]. The main motive to work on this now is because
> of the requirement to have Pinned KVM VMIDs and the RFC discussion
> for the same basically suggested[2] to have a common/better vmid
> allocator for KVM which this series provides.
>  
> Major Changes from v3:
> 
> -Changes related to Pinned ASID support.
> -Changes to take care KPTI related bits reservation.
> -Dropped support for 32 bit KVM.
> -Rebase to 5.12-rc7
> 
> Individual patches have change history for any major changes
> from v3.
> 
> Tests were performed on a HiSilicon D06 platform and so far not observed
> any regressions.
> 
> For ASID allocation,
> 
> Avg of 10 runs(hackbench -s 512 -l 200 -g 300 -f 25 -P),
> 5.12-rc7: Time:18.8119
> 5.12-rc7+v4: Time: 18.459
> 
> ~1.8% improvement.
> 
> For KVM VMID,
> 
> The measurement was made with maxcpus set to 8 and with the
> number of VMID limited to 4-bit. The test involves running
> concurrently 40 guests with 2 vCPUs. Each guest will then
> execute hackbench 5 times before exiting.
> 
> The performance difference between the current algo and the
> new one are(ag. of 10 runs):
>     - 1.9% less exit from the guest
>     - 0.7% faster
> 
> For complete series, please see,
>  https://github.com/hisilicon/kernel-dev/tree/private-v5.12-rc7-asid-v4
> 
> Please take a look and let me know your feedback.

Although I think aligning the two algorithms makes sense, I'm not completely
sold on the need to abstract all this into a library and whether the
additional indirection is justified.

It would be great to compare this approach with one where portions of the
code are duplicated into a separate VMID allocator. Have you tried that to
see what it looks like? Doesn't need to be a proper patch set, but comparing
the end result might help to evaluate the proposal here.

Will