linux-kernel - Re: [PATCH v2 15/18] perf: arm_spe: Handle guest/host exclusion flags

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Date:   Sun, 22 Dec 2019 12:10:52 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Andrew Murray <andrew.murray@....com>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Sudeep Holla <sudeep.holla@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 15/18] perf: arm_spe: Handle guest/host exclusion flags

On Fri, 20 Dec 2019 14:30:22 +0000,
Andrew Murray <andrew.murray@....com> wrote:
> 
> A side effect of supporting the SPE in guests is that we prevent the
> host from collecting data whilst inside a guest thus creating a black-out
> window. This occurs because instead of emulating the SPE, we share it
> with our guests.
> 
> Let's accurately describe our capabilities by using the perf exclude
> flags to prevent !exclude_guest and exclude_host flags from being used.
> 
> Signed-off-by: Andrew Murray <andrew.murray@....com>
> ---
>  drivers/perf/arm_spe_pmu.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> index 2d24af4cfcab..3703dbf459de 100644
> --- a/drivers/perf/arm_spe_pmu.c
> +++ b/drivers/perf/arm_spe_pmu.c
> @@ -679,6 +679,9 @@ static int arm_spe_pmu_event_init(struct perf_event *event)
>  	if (attr->exclude_idle)
>  		return -EOPNOTSUPP;
>  
> +	if (!attr->exclude_guest || attr->exclude_host)
> +		return -EOPNOTSUPP;
> +

I have the opposite approach. If the host decides to profile the
guest, why should that be denied? If there is a black hole, it should
take place in the guest. Today, the host does expect this to work, and
there is no way that we unconditionally allow it to regress.

	M.

-- 
Jazz is not dead, it just smells funny.