linux-kernel - Re: [PATCH v5 07/19] arm64: kvm: Enable access to TRBE support for host

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Date:   Tue, 30 Mar 2021 15:00:16 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        coresight@...ts.linaro.org, mathieu.poirier@...aro.org,
        mike.leach@...aro.org, leo.yan@...aro.org,
        anshuman.khandual@....com, catalin.marinas@....com,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexandru Elisei <Alexandru.Elisei@....com>
Subject: Re: [PATCH v5 07/19] arm64: kvm: Enable access to TRBE support for host

On Tue, 30 Mar 2021 14:34:23 +0100,
Suzuki K Poulose <suzuki.poulose@....com> wrote:
> 
> On 30/03/2021 13:15, Marc Zyngier wrote:
> > On Tue, 30 Mar 2021 12:12:49 +0100,
> > Suzuki K Poulose <suzuki.poulose@....com> wrote:

[...]

> >> May be we could do this check at kvm_arch_vcpu_load()/put() ?
> > 
> > That would extend the tracing blackout period enormously, wouldn't it?
> > I'm not sure that's the best thing to do...
> 
> Sorry for not making this clear. We could check if the SPE/TRBE is
> available on this CPU (including the PMB/TRB_IDR bits and a set a flag
> in the VCPU on every kvm_arch_vcpu_load() and cleared on put.
> The actual switching code could check this flag and check if the unit
> is enabled and then do the actual save/restore as we do below.
> (We may be able to even check if unit is enabled there, need to double
> check this.)

Ah, gotcha. Yes, this seems like a reasonable thing to do. We have the
per-vcpu debug flags already, and you could piggy-back on that.

Thanks,

	M.

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