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

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Date:   Mon, 7 Jun 2021 10:48:55 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>,
        "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>,
        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, Jun 04, 2021 at 04:27:39PM +0100, Marc Zyngier wrote:
> > > 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.
> > 
> > I think this is more related to the "pinned vmid" stuff and was borrowed from
> > the asid_update_limit() fn in arch/arm64/mm/context.c. But I missed the
> > comment that explains the reason behind it. It says,
> > 
> > ---x---
> > 	/*
> > 	 * There must always be an ASID available after rollover. Ensure that,
> > 	 * even if all CPUs have a reserved ASID and the maximum number of ASIDs
> > 	 * are pinned, there still is at least one empty slot in the ASID map.
> > 	 */
> > 	max_pinned_asids = num_available_asids - num_possible_cpus() - 2;
> > ---x---
> > 
> > So this is to make sure we will have at least one VMID available
> > after rollover in case we have pinned the max number of VMIDs. I
> > will include that comment to make it clear.
> 
> That doesn't really explain the -2. Or is that that we have one for
> the extra empty slot, and one for the reserved?
> 
> Jean-Philippe?

Yes, -2 is for ASID#0 and the extra empty slot. A comment higher in
asids_update_limit() hints at that, but it could definitely be clearer

        /*
         * Expect allocation after rollover to fail if we don't have at least
         * one more ASID than CPUs. ASID #0 is reserved for init_mm.
         */

Thanks,
Jean