Re: [Qemu-devel] [PATCH] target-arm: support for ARM1176JZ-s cores

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

On 22 June 2011 00:42, Jamie Iles <jamie@jamieiles.com> wrote:
> On 21 June 2011 23:13, Peter Maydell <peter.maydell@linaro.org> wrote:
>> Ah yes, sorry, I misread the TRM there. So it does have those, it's
>> just the SEV/WFI/WFE it is missing. I guess we'll want an
>> ARM_FEATURE_VAPA too.
>
> Could we perhaps infer and detect some of these features?  For example,
> my reading of the ARM ARM says that the VA<->PA translation registers
> exist for >v7 or v6k if the security extensions exist.  We can detect
> the security extensions from the cpuid registers so we could
> automatically set that feature.

I thought about that, but there are a couple of reasons I'd rather
not detect things from the cpuid/feature registers:
 * older cores don't have them, so you need to cope without them anyway
 * there's a tension between "emulate the same feature regs as the
   h/w" and "emulate feature regs matching what we implement" -- some
   guest OSes will actually refuse to boot unless they get exact matches
   on the feature reg values...
   (At the moment we tend to the former, so we probably advertise the
   security extensions even though we don't implement them, for instance)
 * at the moment the feature regs are just random hex values in helper.c;
   if we wanted to drive things from them we'd need to set up a lot of
   enumerations and constants anyway in order to have something maintainable

Inferring ARM_FEATURE_foo flags from other ARM_FEATURE_foo flags is fine,
though.

Mostly what I'd like is for the actual code implementing things to
be gated on a fairly fine-grained set of flags, so that we can confine
the "what does this core have? what things imply what other things?"
code to a single place where it's easy to tweak if we get it wrong.

-- PMM