linux-kernel - RE: [PATCH v2 1/3] arch/arm64: Introduce a capability to tell whether 16-bit VMID is available

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Message-ID: <027c314e664c4d4889917e95a2aa58c1@huawei.com>
Date:   Thu, 22 Jul 2021 06:24:13 +0000
From:   Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
To:     Will Deacon <will@...nel.org>
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>,
        "maz@...nel.org" <maz@...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@....com" <Alexandru.Elisei@....com>,
        Linuxarm <linuxarm@...wei.com>
Subject: RE: [PATCH v2 1/3] arch/arm64: Introduce a capability to tell whether
 16-bit VMID is available



> -----Original Message-----
> From: Will Deacon [mailto:will@...nel.org]
> Sent: 21 July 2021 16:23
> To: Shameerali Kolothum Thodi <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;
> Alexandru.Elisei@....com; Linuxarm <linuxarm@...wei.com>
> Subject: Re: [PATCH v2 1/3] arch/arm64: Introduce a capability to tell whether
> 16-bit VMID is available
> 
> On Wed, Jun 16, 2021 at 04:56:04PM +0100, Shameer Kolothum wrote:
> > From: Julien Grall <julien.grall@....com>
> >
> > At the moment, the function kvm_get_vmid_bits() is looking up for the
> > sanitized value of ID_AA64MMFR1_EL1 and extract the information
> > regarding the number of VMID bits supported.
> >
> > This is fine as the function is mainly used during VMID roll-over. New
> > use in a follow-up patch will require the function to be called a every
> > context switch so we want the function to be more efficient.
> >
> > A new capability is introduced to tell whether 16-bit VMID is
> > available.
> 
> I don't really buy this rationale. The VMID allocator introduced later on
> caches this value in the static 'vmid_bits' variable, and that gets used
> on vCPU enter via vmid_gen_match() in the kvm_arm_update_vmid() fastpath.
> 
> So I would prefer that we just expose an accessor for that than introduce
> a static key and new cpufeature just for kvm_get_vttbr().

Ok. Will change it to an accessor.

Thanks,
Shameer