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

[Valentin Schneider]

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 :
> 
> ÂÂÂÂ/*
> ÂÂÂÂ * With VHE (HCR.E2H == 1), CPTR_EL2 has the same layout as
> ÂÂÂÂ * CPACR_EL1, except for some missing controls, such as TAM.
> ÂÂÂÂ * And accesses to CPACR_EL1 are routed to CPTR_EL2.
> ÂÂÂÂ * Also CPTR_EL2.TAM has the same position with or without
> ÂÂÂÂ * HCR.E2H == 1. Therefore, use CPTR_EL2.TAM here for
> ÂÂÂÂ * trapping the AMU accesses.
> ÂÂÂÂ */
> 

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