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

[Suzuki Kuruppassery Poulose <suzuki.poulose@arm.com>]

On 27/01/2020 15:33, Valentin Schneider wrote:
> On 18/12/2019 18:26, Ionela Voinescu wrote:
>> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
>> index 6e5d839f42b5..dd20fb185d56 100644
>> --- a/arch/arm64/include/asm/kvm_arm.h
>> +++ b/arch/arm64/include/asm/kvm_arm.h
>> @@ -266,10 +266,11 @@
>>   #define CPTR_EL2_TFP_SHIFT 10
>>   
>>   /* Hyp Coprocessor Trap Register */
>> -#define CPTR_EL2_TCPAC	(1 << 31)
>> -#define CPTR_EL2_TTA	(1 << 20)
>> -#define CPTR_EL2_TFP	(1 << CPTR_EL2_TFP_SHIFT)
>>   #define CPTR_EL2_TZ	(1 << 8)
>> +#define CPTR_EL2_TFP	(1 << CPTR_EL2_TFP_SHIFT)
>> +#define CPTR_EL2_TTA	(1 << 20)
>> +#define CPTR_EL2_TAM	(1 << 30)
>> +#define CPTR_EL2_TCPAC	(1 << 31)
> 
> Nit: why the #define movement? Couldn't that just be added beneath
> CPTR_EL2_TCPAC?
> 
>>   #define CPTR_EL2_RES1	0x000032ff /* known RES1 bits in CPTR_EL2 */
>>   #define CPTR_EL2_DEFAULT	CPTR_EL2_RES1
>>   
>> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
>> index 72fbbd86eb5e..0bca87a2621f 100644
>> --- a/arch/arm64/kvm/hyp/switch.c
>> +++ b/arch/arm64/kvm/hyp/switch.c
>> @@ -90,6 +90,17 @@ static void activate_traps_vhe(struct kvm_vcpu *vcpu)
>>   	val = read_sysreg(cpacr_el1);
>>   	val |= CPACR_EL1_TTA;
>>   	val &= ~CPACR_EL1_ZEN;
>> +
>> +	/*
>> +	 * With VHE enabled, we have HCR_EL2.{E2H,TGE} = {1,1}. Note that in
>> +	 * this case CPACR_EL1 has the same bit layout as CPTR_EL2, and
>> +	 * CPACR_EL1 accessing instructions are redefined to access CPTR_EL2.
>> +	 * Therefore use CPTR_EL2.TAM bit reference to activate AMU register
>> +	 * traps.
>> +	 */
>> +
>> +	val |= CPTR_EL2_TAM;
>> +
> 
> Hmm so this is a bit confusing for me, I've rewritten that part of the
> email too many times (didn't help that I'm far from being a virt guru).
> Rectifications are most welcome.
> 
> 
> First, AFAICT we *don't* have HCR_EL2.TGE set anymore at this point, it's
> cleared just a bit earlier in __activate_traps().
> 
> 
> Then, your comment suggests that when we're running this code, CPACR_EL1
> accesses are rerouted to CPTR_EL2. Annoyingly this isn't mentioned in
> the doc of CPACR_EL1, but D5.6.3 does say
> 
> """
> When ARMv8.1-VHE is implemented, and HCR_EL2.E2H is set to 1, when executing
> at EL2, some EL1 System register access instructions are redefined to access
> the equivalent EL2 register.
> """
> 
> And CPACR_EL1 is part of these, so far so good. Now, the thing is
> the doc for CPACR_EL1 *doesn't* mention any TAM bit - but CPTR_EL2 does.
> I believe what *do* want here is to set CPTR_EL2.TAM (which IIUC we end
> up doing via the rerouting).
> 
> 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.
	 */

Suzuki