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SubjectRe: [PATCH v2 3/6] arm64/kvm: disable access to AMU registers from kvm guests
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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

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