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:

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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!).

quoted

Suzuki