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

[Paul Durrant]

On 23/11/2023 00:07, Volodymyr Babchuk wrote:
> Hi,
>
> Volodymyr Babchuk <volodymyr_babchuk@epam.com> writes:
>
> > Hi Stefano,
> >
> > Stefano Stabellini <sstabellini@kernel.org> 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@epam.com>
> > > > > > > > >
> > > > > > > > > Instead of forcing the owner to domid 0, use XS_PRESERVE_OWNER to
> > > > > > > > > inherit the owner of the directory.
> > > > > > > >
> > > > > > > > Ah... so that's why the previous patch is there.
> > > > > > > >
> > > > > > > > This is not the right way to fix it. The QEMU Xen support is *assuming* that
> > > > > > > > QEMU is either running in, or emulating, dom0. In the emulation case this is
> > > > > > > > probably fine, but the 'real Xen' case it should be using the correct domid
> > > > > > > > for node creation. I guess this could either be supplied on the command line
> > > > > > > > or discerned by reading the local domain 'domid' node.
> > > > > > >
> > > > > > > yes, it should be passed as command line option to QEMU
> > > > > >
> > > > > > I'm not sure I like the idea of a command line option for something
> > > > > > which QEMU could discover for itself.
> > > > >
> > > > > That's fine too. I meant to say "yes, as far as I know the toolstack
> > > > > passes the domid to QEMU as a command line option today".
> > > >
> > > > 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 ?

Actually... is it possible for QEMU just to use a relative path for the 
backend nodes? That way it won't need to know it's own domid, will it?

  Paul