Re: [Xen-devel] [PATCH 1/2] xen/granttable: Support sub-page grants

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 12/06/2011 03:42 AM, Ian Campbell wrote:
>> +{
>> +    int ref;
>> +
>> +    ref = get_free_entries(1);
>> +    if (unlikely(ref < 0))
>> +            return -ENOSPC;
>> +
>> +    gnttab_grant_foreign_access_ref_subpage_v2(ref, domid, frame, flags,
>> +                                               page_off, length);
>> +
>> +    return ref;
>> +}
>> +EXPORT_SYMBOL_GPL(gnttab_grant_foreign_access_subpage_v2);
>> +
>> +void gnttab_grant_foreign_access_ref_subpage_v2(grant_ref_t ref, domid_t 
>> domid,
>> +                                            unsigned long frame, int flags,
>> +                                            unsigned page_off,
>> +                                            unsigned length)
>> +{
>> +    BUG_ON(flags & (GTF_accept_transfer | GTF_reading |
>> +                    GTF_writing | GTF_transitive));
>> +    BUG_ON(grant_table_version == 1);
> Returning -Esomething might be less drastic? ENOSYS perhaps?

Yeah, BUG_ON shouldn't be used for API misuse unless there's absolutely
no other way to handle it.

>
>> +    gnttab_shared.v2[ref].sub_page.frame = frame;
>> +    gnttab_shared.v2[ref].sub_page.page_off = page_off;
>> +    gnttab_shared.v2[ref].sub_page.length = length;
>> +    gnttab_shared.v2[ref].hdr.domid = domid;
>> +    wmb();
>> +    gnttab_shared.v2[ref].hdr.flags =
>> +                            GTF_permit_access | GTF_sub_page | flags;
>> +}
>> +EXPORT_SYMBOL_GPL(gnttab_grant_foreign_access_ref_subpage_v2);
>> +
>> +bool gnttab_subpage_trans_grants_available(void)
>> +{
>> +    return grant_table_version == 2;
>> +}
> It's not clear this adds anything over and above letting the user query
> the grant table version. It's hard to tell since there are no users
> presented here though. Perhaps separate subpage and transitive functions
> would be cleaner?

Well, in general, specifically testing for features rather than
interface versions is better.

    J

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