linux-kernel - Re: [PATCH 1/2] xen/granttable: Support sub-page grants

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Date:	Tue, 06 Dec 2011 09:20:04 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	"annie.li@...cle.com" <annie.li@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"kurt.hackel@...cle.com" <kurt.hackel@...cle.com>,
	Paul Durrant <Paul.Durrant@...rix.com>
Subject: Re: [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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