Re: [Qemu-devel] [PATCH v4 11/21] target-arm: Don't mention PMU in debug feature register

[Peter Maydell <peter.maydell@linaro.org>]

On 17 March 2014 05:13, Peter Crosthwaite <peter.crosthwaite@xilinx.com> wrote:
> On Fri, Mar 7, 2014 at 5:32 AM, Peter Maydell <peter.maydell@linaro.org> wrote:
>> Suppress the ID_AA64DFR0_EL1 PMUVer field, even if the CPU specific
>> value claims that it exists. QEMU doesn't currently implement it,
>> and not advertising it prevents the guest from trying to use it
>> and getting UNDEFs on unimplemented registers.
>>
>> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
>
> Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
>
>> ---
>> This is arguably a hack, but otherwise Linux tries to prod
>> half a dozen PMU sysregs.
>
> Not really. I think sane self-identification trumps dummy feature
> advertising. Although there is a consistency argument to be made, as
> to whether you should also wipe-out any other features advertised by
> this register and friends (e.g. should TraceVer be set?).

The lack of consistency is what makes it a hack :-) Generally
QEMU takes the approach of "report what the h/w reports even
if we don't implement it all"; "report what we provide even
if that's not the same values as h/w" would be a different
approach, but if we wanted that we'd need to do it consistently.
Still I think pragmatism wins out here.

thanks
-- PMM