Re: [Qemu-devel] [PATCH 1/3] kvm: check KVM_CAP_SYNC_MMU with kvm_vm_check_extension()

[Thomas Huth <thuth@redhat.com>]

On 14.09.2017 21:25, Greg Kurz wrote:
> On a server-class ppc host, this capability depends on the KVM type,
> ie, HV or PR. If both KVM are present in the kernel, we will always
> get the HV specific value, even if we explicitely requested PR on
> the command line.
> 
> This can have an impact if we're using hugepages or a balloon device.
> 
> Since we've already created the VM at the time any user calls
> kvm_has_sync_mmu(), switching to kvm_vm_check_extension() is
> enough to fix any potential issue.
> 
> It is okay for the other archs that also implement KVM_CAP_SYNC_MMU,
> ie, mips, s390, x86 and arm, because they don't depend on the VM being
> created or not.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>  accel/kvm/kvm-all.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index f85553a85194..323c567cfb68 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -2234,7 +2234,7 @@ int kvm_device_access(int fd, int group, uint64_t attr,
>  /* Return 1 on success, 0 on failure */
>  int kvm_has_sync_mmu(void)
>  {
> -    return kvm_check_extension(kvm_state, KVM_CAP_SYNC_MMU);
> +    return kvm_vm_check_extension(kvm_state, KVM_CAP_SYNC_MMU);
>  }

Reviewed-by: Thomas Huth <thuth@redhat.com>

... but while you're at it, maybe it would be better to use a bool
variable for the state of this extension, too, and only check for the
extension one time at the end of kvm_init() ? kvm_has_sync_mmu() is
apparently used multiple times in other source files, so we might be
able to save some cycles by doing the syscall only once?

 Thomas