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


From: David Woodhouse
Subject: Re: [PATCH v2 4/6] xen_pvdev: Do not assume Dom0 when creating a directory
Date: Thu, 23 Nov 2023 10:45:47 +0000
User-agent: Evolution 3.44.4-0ubuntu2

On Thu, 2023-11-23 at 09:28 +0000, Paul Durrant wrote:
> 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?

That covers some of the use cases, but it may also need to know its own
domid for other purposes. Including writing the *absolute* path of the
backend, to a frontend node?

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