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

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Message-ID: <5de3d3c7-2d78-9d18-f3ca-7cb6cf9ce36c@arm.com>
Date:   Tue, 28 Jan 2020 17:37:04 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Suzuki Kuruppassery Poulose <suzuki.poulose@....com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        catalin.marinas@....com, will@...nel.org, mark.rutland@....com,
        maz@...nel.org, sudeep.holla@....com, dietmar.eggemann@....com
Cc:     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

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 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