linux-kernel - Re: [RFC V2 3/3] perf: qcom: Add Falkor CPU PMU IMPLEMENTATION DEFINED event support

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Date:   Wed, 13 Jun 2018 13:59:58 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Will Deacon <will.deacon@....com>,
        Agustin Vega-Frias <agustinv@...eaurora.org>
Cc:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Jeremy Linton <jeremy.linton@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        timur@...eaurora.org
Subject: Re: [RFC V2 3/3] perf: qcom: Add Falkor CPU PMU IMPLEMENTATION
 DEFINED event support

On 13/06/18 11:35, Will Deacon wrote:
> On Tue, Jun 12, 2018 at 04:41:32PM -0400, Agustin Vega-Frias wrote:
>> Hi Mark,
>>
>> On 2018-06-12 10:40, Mark Rutland wrote:
>>> Hi,
>>>
>>> On Thu, Jun 07, 2018 at 09:56:48AM -0400, Agustin Vega-Frias wrote:
>>>> Selection of these events can be envisioned as indexing them from
>>>> a 3D matrix:
>>>> - the first index selects a Region Event Selection Register
>>>> (PMRESRx_EL0)
>>>> - the second index selects a group from which only one event at a time
>>>>  can be selected
>>>> - the third index selects the event
>>>>
>>>> The event is encoded into perf_event_attr.config as 0xPRCCG, where:
>>>>  P  [config:16   ] = prefix   (flag that indicates a matrix-based
>>>> event)
>>>>  R  [config:12-15] = register (specifies the PMRESRx_EL0 instance)
>>>>  G  [config:0-3  ] = group    (specifies the event group)
>>>>  CC [config:4-11 ] = code     (specifies the event)
>>>>
>>>> Events with the P flag set to zero are treated as common PMUv3 events
>>>> and are directly programmed into PMXEVTYPERx_EL0.
>>>>
>>>> The first two indexes are set combining the RESR and group number with
>>>> a base number and writing it into the architected PMXEVTYPER_EL0
>>>> register.
>>>> The third index is set by writing the code into the bits corresponding
>>>> with the group into the appropriate IMPLEMENTATION DEFINED PMRESRx_EL0
>>>> register.
>>>
>>> When are the IMP DEF registers accessible at EL0? Are those goverend by
>>> the same controls as the architected registers?
>>
>> No, there is a separate IMP DEF register to control access.
> 
> Great :( We need to make sure we disable EL0 access during boot then, but
> that means we need to prove for the existence of this thing in head.S
> (since the PMU driver might not get loaded).
> 
> Also, what's the kvm story here so that we don't accidentally open up a
> VM-VM side-channel via these registers? How do the EL1 trapping controls
> work?

We'd trap the IMPDEF register access and inject an UNDEF (assuming that
the IMPDEF trapping works correctly). I have strictly no plan to support
this in a guest.

Thanks,

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