Re: [Qemu-devel] [kvm-unit-tests PATCH v13 2/4] arm: Add PMU test

[Peter Maydell]

On 1 December 2016 at 12:19, Andre Przywara <address@hidden> wrote:
> Hi,
>
> On 01/12/16 12:02, Peter Maydell wrote:
>> On 1 December 2016 at 11:28, Andre Przywara <address@hidden> wrote:
>>> I don't think so. At least here as a _variable_ type uint32_t is
>>> probably the right one, as the ARMv8 ARM explicitly says that PMCR is a
>>> 32-bit register, for both bitnesses.
>>
>> For 64-bit ARM this is strictly speaking just shorthand for "64-bit
>> register with the top 32-bit being RES0". It is in theory possible that
>> a future architecture extension might define uses for those RES0
>> bits.
>
> I trade: "in theory possible that a future architecture extension might"
> (that's four speculative terms, right?) against:
>
> ARMv8 ARM, D7.4.7  PMCR_EL0, Performance Monitors Control Register:
> Attributes
>             PMCR_EL0 is a 32-bit register.

As I say, this just means "64 bit with 32 RES0 bits". See DDI0487A.k
C5.1.1 "System register width":

# In AArch64 state, each encoding in the System instruction space can
# provide access to a 64-bit register. An AArch64 System register is
# described as either a 32-bit register or a 64-bit register. For
# a 32-bit register, the upper bits, bits[63:32], are RES0.

(ie the register is 64-bits, it's just "described as" 32-bits for
convenience.)

thanks
-- PMM