linux-kernel - Re: [PATCH 8/8] arm64: cpufeature: Add an overview comment for the cpufeature framework

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Date:   Thu, 16 Apr 2020 16:26:05 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     will@...nel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        mark.rutland@....com, anshuman.khandual@....com,
        catalin.marinas@....com, saiprakash.ranjan@...eaurora.org,
        dianders@...omium.org, kernel-team@...roid.com
Subject: Re: [PATCH 8/8] arm64: cpufeature: Add an overview comment for the
 cpufeature framework

On 2020-04-16 15:59, Suzuki K Poulose wrote:

Hi Suzuki,

[...]

> As you mentioned in the other response we could add information about
> the guest view, something like :
> 
>       - KVM exposes the sanitised value of the feature registers to the
> 	guests and is not affected by the FTR_VISIBLE. However,
> 	depending on the individual feature support in the hypervisor,
> 	some of the fields may be capped/limited.

Although in most cases, what KVM exposes is indeed a strict subset of
the host's features, there is a few corner cases where we expose 
features
that do not necessarily exist on the host. For example ARMv8.5-GTG and
ARMv8.4-TTL get exposed by the NV patches even if they don't exist on 
the
host, as KVM will actually emulate them.

Not a big deal, but I just wanted to outline that it isn't as clear-cut 
as
it may seem...

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...