Re: [Qemu-devel] [PATCH] target-i386: kvm: cache KVM_GET_SUPPORTED_CPUID data

[Paolo Bonzini]

On 13/06/2016 04:21, Chao Peng wrote:
> KVM_GET_SUPPORTED_CPUID ioctl is called frequently when initializing
> CPU. Depends on CPU features and CPU count, the number of calls can be
> extremely high which slows down QEMU booting significantly. In our
> testing, we saw 5922 calls with switches:
> 
>     -cpu SandyBridge -smp 6,sockets=6,cores=1,threads=1
> 
> This ioctl takes more than 100ms, which is almost half of the total
> QEMU startup time.
> 
> While for most cases the data returned from two different invocations
> are not changed, that means, we can cache the data to avoid trapping
> into kernel for the second time. To make sure the cache safe one
> assumption is desirable: the ioctl is stateless. This is not true
> however, at least for some CPUID leaves.

Which are the CPUID leaves for which KVM_GET_SUPPORTED_CPUID is not
stateless?  I cannot find any.

> The good part is even the ioctl is not fully stateless, we can still
> cache the return value if we know the data is unchanged for the leaves
> we are interested in. Actually this should be true for most invocations
> and looks all the places in current code hold true.
> 
> A non-cached version can be introduced if refresh is required in the
> future.

[...]

> 
> +static Notifier kvm_exit_notifier;
> +static void kvm_arch_destroy(Notifier *n, void *unused)
> +{
> +    g_free(cpuid_cache);
> +}
> +
>  int kvm_arch_init(MachineState *ms, KVMState *s)
>  {
>      uint64_t identity_base = 0xfffbc000;
> @@ -1165,6 +1176,9 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>          smram_machine_done.notify = register_smram_listener;
>          qemu_add_machine_init_done_notifier(&smram_machine_done);
>      }
> +
> +    kvm_exit_notifier.notify = kvm_arch_destroy;
> +    qemu_add_exit_notifier(&kvm_exit_notifier);
>      return 0;


This part is unnecessary; the OS takes care of freeing the heap on exit.

Thanks,

Paolo