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From: Ionela Voinescu <ionela.voinescu@arm.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Suzuki Kuruppassery Poulose <suzuki.poulose@arm.com>,
	catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com,
	maz@kernel.org, sudeep.holla@arm.com, dietmar.eggemann@arm.com,
	peterz@infradead.org, mingo@redhat.com, ggherdovich@suse.cz,
	vincent.guittot@linaro.org, linux-arm-kernel@lists.infradead.org,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>
Subject: Re: [PATCH v2 3/6] arm64/kvm: disable access to AMU registers from kvm guests
Date: Tue, 28 Jan 2020 17:52:23 +0000	[thread overview]
Message-ID: <20200128175223.GA31241@arm.com> (raw)
In-Reply-To: <5de3d3c7-2d78-9d18-f3ca-7cb6cf9ce36c@arm.com>

On Tuesday 28 Jan 2020 at 17:37:04 (+0000), Valentin Schneider wrote:
> Hi Suzuki,
> 
> On 28/01/2020 17:26, Suzuki Kuruppassery Poulose wrote:
> >> So, providing I didn't get completely lost on the way, I have to ask:
> >> why do we use CPACR_EL1 here? Couldn't we use CPTR_EL2 directly?
> > 
> > Part of the reason is, CPTR_EL2 has different layout depending on
> > whether HCR_EL2.E2H == 1. e.g, CPTR_EL2.TTA move from Bit[28] to Bit[20].
> > 
> > So, to keep it simple, CPTR_EL2 is used for non-VHE code with the shifts
> > as defined by the "CPTR_EL2 when E2H=0"
> > 
> > if E2H == 1, CPTR_EL2 takes the layout of CPACR_EL1 and "overrides" some
> > of the RES0 bits in CPACR_EL1 with EL2 controls (e.g: TAM, TCPAC).
> > Thus we use CPACR_EL1 to keep the "shifts" non-conflicting (e.g, ZEN)
> > and is the right thing to do.
> > 
> > It is a bit confusing, but we are doing the right thing. May be we could improve the comment like :
> > 
> >     /*
> >      * With VHE (HCR.E2H == 1), CPTR_EL2 has the same layout as
> >      * CPACR_EL1, except for some missing controls, such as TAM.
> >      * And accesses to CPACR_EL1 are routed to CPTR_EL2.
> >      * Also CPTR_EL2.TAM has the same position with or without
> >      * HCR.E2H == 1. Therefore, use CPTR_EL2.TAM here for
> >      * trapping the AMU accesses.
> >      */
> >

Thanks Suzuki, this makes sense!

Ionela.

> 
> Thanks for clearing this up! I also bothered MarcZ in the meantime who
> also cleared up some of my confusion (including which layout takes effect).
> 
> So yeah, I think what we want here is to keep using CPTR_EL2_TAM but have a
> comment that explains why (which you just provided!).
> 
> > Suzuki

  reply	other threads:[~2020-01-28 17:52 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-18 18:26 [PATCH v2 0/6] arm64: ARMv8.4 Activity Monitors support Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 1/6] arm64: add support for the AMU extension v1 Ionela Voinescu
2020-01-23 17:04   ` Valentin Schneider
2020-01-23 18:32     ` Ionela Voinescu
2020-01-24 12:00       ` Valentin Schneider
2020-01-28 11:00         ` Ionela Voinescu
2020-01-28 16:34   ` Suzuki Kuruppassery Poulose
2020-01-29 16:42     ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 2/6] arm64: trap to EL1 accesses to AMU counters from EL0 Ionela Voinescu
2020-01-23 17:04   ` Valentin Schneider
2020-01-23 17:34     ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 3/6] arm64/kvm: disable access to AMU registers from kvm guests Ionela Voinescu
2020-01-27 15:33   ` Valentin Schneider
2020-01-28 15:48     ` Ionela Voinescu
2020-01-28 17:26     ` Suzuki Kuruppassery Poulose
2020-01-28 17:37       ` Valentin Schneider
2020-01-28 17:52         ` Ionela Voinescu [this message]
2019-12-18 18:26 ` [PATCH v2 4/6] Documentation: arm64: document support for the AMU extension Ionela Voinescu
2020-01-27 16:47   ` Valentin Schneider
2020-01-28 16:53     ` Ionela Voinescu
2020-01-28 18:36       ` Valentin Schneider
2020-01-30 15:04   ` Suzuki Kuruppassery Poulose
2020-01-30 16:45     ` Ionela Voinescu
2020-01-30 18:26       ` Suzuki K Poulose
2020-01-31  9:54         ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 5/6] TEMP: sched: add interface for counter-based frequency invariance Ionela Voinescu
2020-01-29 19:37   ` Peter Zijlstra
2020-01-30 15:33     ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 6/6] arm64: use activity monitors for " Ionela Voinescu
2020-01-23 11:49   ` Lukasz Luba
2020-01-23 17:07     ` Ionela Voinescu
2020-01-24  1:19       ` Lukasz Luba
2020-01-24 13:12         ` Ionela Voinescu
2020-01-24 15:17           ` Lukasz Luba
2020-01-28 17:36             ` Ionela Voinescu
2020-01-29 17:13   ` Valentin Schneider
2020-01-29 17:52     ` Ionela Voinescu
2020-01-29 23:39     ` Valentin Schneider
2020-01-30 15:49       ` Ionela Voinescu
2020-01-30 16:11         ` Valentin Schneider

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