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

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

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 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.

-- 
Any sufficiently difficult bug is indistinguishable from a feature.


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