linux-kernel - Re: [PATCH v2 3/6] arm64/kvm: disable access to AMU registers from kvm guests

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Date:   Tue, 28 Jan 2020 17:52:23 +0000
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Suzuki Kuruppassery Poulose <suzuki.poulose@....com>,
        catalin.marinas@....com, will@...nel.org, mark.rutland@....com,
        maz@...nel.org, sudeep.holla@....com, dietmar.eggemann@....com,
        peterz@...radead.org, mingo@...hat.com, ggherdovich@...e.cz,
        vincent.guittot@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>
Subject: Re: [PATCH v2 3/6] arm64/kvm: disable access to AMU registers from
 kvm guests

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