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SubjectRe: [PATCH 1/2] xen/granttable: Support sub-page grants
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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