Re: [Xen-devel] [PATCH RFC 08/31] tools/stubs: Expose host featureset to userspace

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/01/16 15:36, Ian Campbell wrote:
> On Wed, 2015-12-16 at 21:24 +0000, Andrew Cooper wrote:
>> diff --git a/tools/libxc/xc_misc.c b/tools/libxc/xc_misc.c
>> index c613545..4d7af3d 100644
>> --- a/tools/libxc/xc_misc.c
>> +++ b/tools/libxc/xc_misc.c
>> @@ -718,6 +718,33 @@ int xc_hvm_inject_trap(
>>      return rc;
>>  }
>>  
>> +int xc_get_featureset(xc_interface *xch, uint32_t index,
>> +                      uint32_t *nr_features, uint32_t *featureset)
> This looks like a valid binding to the hypercall iface, so once that is
> agreed this LGTM.
>
>> diff --git a/tools/ocaml/libs/xc/xenctrl_stubs.c
>> b/tools/ocaml/libs/xc/xenctrl_stubs.c
>> index b7de615..a47473b 100644
>> --- a/tools/ocaml/libs/xc/xenctrl_stubs.c
>> +++ b/tools/ocaml/libs/xc/xenctrl_stubs.c
>> @@ -1220,6 +1220,41 @@ CAMLprim value
>> stub_xc_domain_deassign_device(value xch, value domid, value desc
>>      CAMLreturn(Val_unit);
>>  }
>>  
>> +CAMLprim value stub_xc_get_featureset(value xch, value idx)
>> +{
>> +    CAMLparam2(xch, idx);
>> +    CAMLlocal1(bitmap_val);
>> +
>> +    /* Safe, because of the global ocaml lock. */
>> +    static uint32_t fs_len;
>> +
>> +    if (fs_len == 0)
>> +    {
>> +            int ret = xc_get_featureset(_H(xch), 0, &fs_len, NULL);
>> +
>> +            if (ret || (fs_len == 0))
>> +                    failwith_xc(_H(xch));
>> +    }
> This confuses me because I had thought when reading the previous patch that
> the output nr_features would depend on the specific index, is that not the
> case? Maybe this is just a hypercall docs fix?

nr_features is compile-time-constant in Xen, and every featureset handed
back will be of that length.

It is expected to grow given new changes to Xen, but won't change at
runtime.

~Andrew

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