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Re: [PATCH] xen: add option to disable GNTTABOP_transfer
- To: Juergen Gross <jgross@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 3 Feb 2022 13:04:29 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 03 Feb 2022 12:04:45 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 03.02.2022 11:55, Juergen Gross wrote:
> On 03.02.22 10:10, Jan Beulich wrote:
>> On 01.02.2022 10:02, Juergen Gross wrote:
>>> The grant table operation GNTTABOP_transfer is meant to be used in
>>> PV device backends, and it hasn't been used in Linux since the old
>>> Xen-o-Linux days.
>>
>> Kind of unusual spelling of XenoLinux ;-)
>>
>>> --- a/xen/common/grant_table.c
>>> +++ b/xen/common/grant_table.c
>>> @@ -181,6 +181,7 @@ static int parse_gnttab_max_maptrack_frames(const char
>>> *arg)
>>>
>>> unsigned int __read_mostly opt_gnttab_max_version = GNTTAB_MAX_VERSION;
>>> static bool __read_mostly opt_transitive_grants = true;
>>> +static bool __read_mostly opt_grant_transfer = true;
>>
>> If this was conditional upon PV (with a #define to false in the
>> opposite case), it could be __ro_after_init right away, while at
>> the same time allowing the compiler to eliminate gnttab_transfer().
>
> Nice idea. The other option would be to put all (or most) of
> gnttab_transfer() in a "#ifdef CONFIG_PV" section, allowing to
> remove the "#ifdef CONFIG_X86" parts in it, too.
Yes, sure. The downside being that then this code won't be compile-
tested anymore in !PV builds. Yet keeping code visible to compilers
is what we aim for elsewhere by preferring if(IS_ENABLED(...)) over
#ifdef where possible.
Jan
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