Re: [PATCH for-4.14] mm: fix public declaration of struct xen_mem_acquire_resource

[Julien Grall <julien@xxxxxxx>]

Hi,

On 24/06/2020 13:08, Jan Beulich wrote:
> On 24.06.2020 12:52, Julien Grall wrote:
> > Hi Jan,
> >
> > On 24/06/2020 11:05, Jan Beulich wrote:
> > > On 23.06.2020 19:32, Roger Pau Monné wrote:
> > > > On Tue, Jun 23, 2020 at 05:04:53PM +0200, Jan Beulich wrote:
> > > > > On 23.06.2020 15:52, Roger Pau Monne wrote:
> > > > > > XENMEM_acquire_resource and it's related structure is currently inside
> > > > > > a __XEN__ or __XEN_TOOLS__ guarded section to limit it's scope to the
> > > > > > hypervisor or the toolstack only. This is wrong as the hypercall is
> > > > > > already being used by the Linux kernel at least, and as such needs to
> > > > > > be public.
> > > > > Actually - how does this work for the Linux kernel, seeing
> > > > >
> > > > >       rc = rcu_lock_remote_domain_by_id(xmar.domid, &d);
> > > > >       if ( rc )
> > > > >           return rc;
> > > > >
> > > > >       rc = xsm_domain_resource_map(XSM_DM_PRIV, d);
> > > > >       if ( rc )
> > > > >           goto out;
> > > > >
> > > > > in the function?
> > > > It's my understanding (I haven't tried to use that hypercall yet on
> > > > FreeBSD, so I cannot say I've tested it), that xmar.domid is the
> > > > remote domain, which the functions locks and then uses
> > > > xsm_domain_resource_map to check whether the current domain has
> > > > permissions to do privileged operations against it.
> > > Yes, but that's a tool stack operation, not something the kernel
> > > would do all by itself. The kernel would only ever pass DOMID_SELF
> > > (or the actual local domain ID), I would think.
> > You can't issue that hypercall directly from userspace because you need
> > to map the page in the physical address space of the toolstack domain.
> >
> > So the kernel has to act as the proxy for the hypercall. This is
> > implemented as mmap() in Linux.
> Oh, and there's no generic wrapping available here, unlike for
> dmop. 
It is not clear to me the sort of generic wrapping you are referring to. 
Are you referring to a stable interface for an application?
> Makes me wonder whether, for this purpose, there should
> be (have been) a new dmop with identical functionality, to
> allow such funneling.
I am not sure how using DMOP will allow us to implement it fully in 
userspace. Do you mind expanding it?
Cheers,

--
Julien Grall