Re: [Qemu-devel] [PATCH] kvm: Fix enable_cap helpers on older gcc

[Thomas Huth <thuth@linux.vnet.ibm.com>]

On Sun, 11 May 2014 18:11:04 +0200
Alexander Graf <agraf@suse.de> wrote:

> Commit 40f1ee27aa1 introduced handy helpers for enable_cap calls on
> vcpu and vm level. Unfortunately some older gcc versions (4.7.1, 4.6)
> seem to choke on signedness detection in inline created variables:
> 
> target-ppc/kvm.c: In function 'kvmppc_booke_watchdog_enable':
> target-ppc/kvm.c:1302:21: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]
> target-ppc/kvm.c: In function 'kvmppc_set_papr':
> target-ppc/kvm.c:1504:21: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]
> 
> We can easily work around this by not doing a compare with 0, but instead
> shift all values up by one. The resulting binary should look the same, as
> all those ARRAY_SIZE values are compile time statically optimized.
> 
> This patch fixes compile errors on some of my systems.
> 
> Signed-off-by: Alexander Graf <agraf@suse.de>
> ---
>  include/sysemu/kvm.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
> index 192fe89..989c261 100644
> --- a/include/sysemu/kvm.h
> +++ b/include/sysemu/kvm.h
> @@ -302,9 +302,9 @@ int kvm_check_extension(KVMState *s, unsigned int extension);
>          };                                                           \
>          uint64_t args_tmp[] = { __VA_ARGS__ };                       \
>          int i;                                                       \
> -        for (i = 0; i < ARRAY_SIZE(args_tmp) &&                      \
> -                     i < ARRAY_SIZE(cap.args); i++) {                \
> -            cap.args[i] = args_tmp[i];                               \
> +        for (i = 1; i < (MIN(ARRAY_SIZE(args_tmp),                   \
> +                             ARRAY_SIZE(cap.args)) + 1); i++) {      \
> +            cap.args[i - 1] = args_tmp[i - 1];                       \
>          }                                                            \
>          kvm_vm_ioctl(s, KVM_ENABLE_CAP, &cap);                       \
>      })
> @@ -317,9 +317,9 @@ int kvm_check_extension(KVMState *s, unsigned int extension);
>          };                                                           \
>          uint64_t args_tmp[] = { __VA_ARGS__ };                       \
>          int i;                                                       \
> -        for (i = 0; i < ARRAY_SIZE(args_tmp) &&                      \
> -                     i < ARRAY_SIZE(cap.args); i++) {                \
> -            cap.args[i] = args_tmp[i];                               \
> +        for (i = 1; i < (MIN(ARRAY_SIZE(args_tmp),                   \
> +                             ARRAY_SIZE(cap.args)) + 1); i++) {      \
> +            cap.args[i - 1] = args_tmp[i - 1];                       \
>          }                                                            \
>          kvm_vcpu_ioctl(cpu, KVM_ENABLE_CAP, &cap);                   \
>      })

That looks ... somewhat ugly. I think you either should add a proper
comment here (or somebody will fix it back in the future when reading
the code), ... or would it also work to type-cast the result of
ARRAY_SIZE to "int"? Something like this:

        for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&                   \
                     i < (int)ARRAY_SIZE(cap.args); i++)

 Thomas