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

[Gleb Natapov <gleb@redhat.com>]

On Sun, Aug 25, 2013 at 04:11:51PM +0100, Peter Maydell wrote:
> On 25 August 2013 15:42, Gleb Natapov <gleb@redhat.com> 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.
That's sad. If feature bit is available I would expect well behaved
guest to test it. BTW is there architectural way to tell OS that it
runs as a guest? There is dedicated CPUID bit on x86 for that which is
obviously never set on real CPUs.

>                       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).
What if you want to emulate older arm arch for a guest?

>                   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.)
> 
Do we have a way to ask KVM if it can support those features?

--
			Gleb.