Re: [PATCH v2 4/6] xen_pvdev: Do not assume Dom0 when creating a directory

[Alex Bennée <alex.bennee@xxxxxxxxxx>]

Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx> writes:

> Hi,
>
> Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx> writes:
>
>> Hi Stefano,
>>
>> Stefano Stabellini <sstabellini@xxxxxxxxxx> writes:
>>
>>> On Wed, 22 Nov 2023, David Woodhouse wrote:
>>>> On Wed, 2023-11-22 at 15:09 -0800, Stefano Stabellini wrote:
>>>> > On Wed, 22 Nov 2023, David Woodhouse wrote:
>>>> > > On Wed, 2023-11-22 at 14:29 -0800, Stefano Stabellini wrote:
>>>> > > > On Wed, 22 Nov 2023, Paul Durrant wrote:
>>>> > > > > On 21/11/2023 22:10, Volodymyr Babchuk wrote:
>>>> > > > > > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
<snip>
>>>> The -xen-domid argument on the QEMU command line today is the *guest*
>>>> domain ID, not the domain ID in which QEMU itself is running.
>>>> 
>>>> Or were you thinking of something different?
>>>
>>> Ops, you are right and I understand your comment better now. The backend
>>> domid is not on the command line but it should be discoverable (on
>>> xenstore if I remember right).
>>
>> Yes, it is just "~/domid". I'll add a function that reads it.
>
> Just a quick question to QEMU folks: is it better to add a global
> variable where we will store own Domain ID or it will be okay to read
> domid from Xenstore every time we need it?
>
> If global variable variant is better, what is proffered place to define
> this variable? system/globals.c ?

Hmm maybe, I see Xen already has some but the comment:

  "Global variables that (mostly) should not exist"

I think it to dissuade the file growing more than it should.

I think generally the best pattern to use if a global can't be avoided
is to have a "static global" in the main .c file for the module and then
provide a helper access function for other files to read it. That also
makes re-factoring easier if things like locking need to be added down
the line.

We still do have a few true global variables which need "extern"
declarations in the headers but if we can avoid adding more that would
be good.

Of course ideally this sort of data would be wrapped up in QOM
structures but I can see the argument for the host domain ID.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro