Re: [Qemu-devel] [RFC 0/2] target-arm: Provide '-cpu host' when running KVM

[Peter Maydell]

On 25 August 2013 15:42, Gleb Natapov <address@hidden> wrote:
> Are ARM cpu features not enumerable and the only way to know what is
> supported by a core is by its id? I do see feature registers in the
> spec, so why wouldn't we want to be able to disable/enable features
> individually?

It's complicated. There are feature registers which have various
bits, but only some combinations are architecturally valid. In any
case a guest might reasonably legitimately assume "this is an A15
so it will have architected timers", for example, and not bother
testing feature bits. Some feature bits really do have to match the
real hardware, like ones that say "you need to explicitly flush the
branch predictor". A lot are feature bits that are simply mandatory
for any ARMv7 or later processor (which is what you need for KVM
to work at all). And buried in there might possibly be a few features
that it might actually make sense to enable/disable separately (like
"do we have Neon?").

So what we have for now is that the INIT_VCPU ioctl has space
for feature flags (currently unused in v7; in v8 there's a "make the
VM be a 32 bit VM on a 64 bit CPU" flag), so we could wire those
up some day. (We don't have any support in QEMU for saying
"I want an ARM CPU with features X and Y but not Z" yet either.)

-- PMM