linux-kernel - Re: [RFC PATCH 0/3] kvm/arm: New VMID allocator based on asid(2nd approach)

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Date:   Fri, 04 Jun 2021 14:54:37 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "will@...nel.org" <will@...nel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "james.morse@....com" <james.morse@....com>,
        "julien.thierry.kdev@...il.com" <julien.thierry.kdev@...il.com>,
        "suzuki.poulose@....com" <suzuki.poulose@....com>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        Alexandru Elisei <Alexandru.Elisei@....com>,
        Linuxarm <linuxarm@...wei.com>
Subject: Re: [RFC PATCH 0/3] kvm/arm: New VMID allocator based on asid(2nd approach) 

On Fri, 04 Jun 2021 09:13:10 +0100,
Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com> wrote:
> 
> Hi,
> 
> > -----Original Message-----
> > From: Shameerali Kolothum Thodi
> > Sent: 06 May 2021 17:52
> > To: linux-arm-kernel@...ts.infradead.org; kvmarm@...ts.cs.columbia.edu;
> > linux-kernel@...r.kernel.org
> > Cc: maz@...nel.org; will@...nel.org; catalin.marinas@....com;
> > james.morse@....com; julien.thierry.kdev@...il.com;
> > suzuki.poulose@....com; jean-philippe@...aro.org; Linuxarm
> > <linuxarm@...wei.com>
> > Subject: [RFC PATCH 0/3] kvm/arm: New VMID allocator based on asid(2nd
> > approach)
> > 
> > This is based on a suggestion from Will [0] to try out the asid
> > based kvm vmid solution as a separate VMID allocator instead of
> > the shared lib approach attempted in v4[1].
> > 
> > The idea is to compare both the approaches and see whether the
> > shared lib solution with callbacks make sense or not.
> 
> A gentle ping on this. Please take a look and let me know.

I had a look and I don't overly dislike it. I'd like to see the code
without the pinned stuff though, at least to ease the reviewing. I
haven't tested it in anger, but I have pushed the rebased code at [1]
as it really didn't apply to 5.13-rc4.

One thing I'm a bit worried about is that we so far relied on VMID 0
never being allocated to a guest, which is now crucial for protected
KVM. I can't really convince myself that this can never happen with
this. Plus, I've found this nugget:

<quote
	max_pinned_vmids = NUM_USER_VMIDS - num_possible_cpus() - 2;
</quote>

What is this "- 2"? My hunch is that it should really be "- 1" as VMID
0 is reserved, and we have no equivalent of KPTI for S2.

	M.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/mmu/vmid

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