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Date:   Wed, 21 Jul 2021 16:23:17 +0100
From:   Will Deacon <will@kernel.org>
To:     Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Cc:     linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-kernel@vger.kernel.org, maz@kernel.org,
        catalin.marinas@arm.com, james.morse@arm.com,
        julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com,
        jean-philippe@linaro.org, Alexandru.Elisei@arm.com,
        linuxarm@huawei.com
Subject: Re: [PATCH v2 1/3] arch/arm64: Introduce a capability to tell
 whether 16-bit VMID is available
Message-ID: <20210721152316.GB11003@willie-the-truck>
References: <20210616155606.2806-1-shameerali.kolothum.thodi@huawei.com>
 <20210616155606.2806-2-shameerali.kolothum.thodi@huawei.com>
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On Wed, Jun 16, 2021 at 04:56:04PM +0100, Shameer Kolothum wrote:
> From: Julien Grall <julien.grall@arm.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().

Will