Re: [PATCH 3/3] Add the paravirtualization CPUID entry to QEMU (v2)

[Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

On Mon, 2007-08-27 at 22:54 +0300, Avi Kivity wrote:
> Anthony Liguori wrote:
> > This adds a CPUID entry for the paravirtualization feature bitmap.  We can do
> > this unconditionally because the guest requires that both the feature CPUID
> > entry and the signature CPUID entry exists to enable paravirtualization.
> >
> > This means that guest will never enable paravirtualization if either userspace
> > or kernelspace doesn't support paravirtualization.
> >
> > Signed-off-by: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
> >
> > diff --git a/qemu/qemu-kvm.c b/qemu/qemu-kvm.c
> > index 491c32c..1c93388 100644
> > --- a/qemu/qemu-kvm.c
> > +++ b/qemu/qemu-kvm.c
> > @@ -1103,12 +1103,18 @@ static void do_cpuid_ent(struct kvm_cpuid_entry *e, uint32_t function,
> >  int kvm_qemu_init_env(CPUState *cenv)
> >  {
> >      struct kvm_cpuid_entry cpuid_ent[100];
> > +    struct kvm_cpuid_entry *pv_features;
> >      int cpuid_nent = 0;
> >      CPUState copy;
> >      uint32_t i, limit;
> >  
> >      copy = *cenv;
> >  
> > +    pv_features = &cpuid_ent[cpuid_nent++];
> > +    memset(pv_features, 0, sizeof(*pv_features));
> > +    pv_features->function = 0x40000003;
> > +    pv_features->eax = (1 << KVM_PARA_FEATURE_NOP_IO_DELAY);
> > +
> >      copy.regs[R_EAX] = 0;
> >      qemu_kvm_cpuid_on_env(&copy);
> >      limit = copy.regs[R_EAX];
> >   
> 
> Shouldn't we query kvm here to see if the feature is needed?

I don't think so.  At the moment, PIO goes to QEMU so QEMU is really the
only one that should be making that decision.

> I imagine I/O delay is never needed unless we passthrough a device, 
> which this qemu doesn't support, but it would be nice to set a good 
> example for the future.

In this particular case, it would be a bad example since the kernel
isn't the one that ought to be making the decision.  I suspect PCI
pass-through should be initiated in QEMU anyway so it still will be the
one making the decision.

However, if you apply these patches before my flight levels tomorrow, I
can send out the MMU optimization patches that include a proper example
of how to query kernel-space ;-)

Regards,

Anthony Liguori



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