Re: Uses of /hypervisor memory range (was: FreeBSD/Xen/ARM issues)

From: "Roger Pau Monné" <royger@freebsd.org>
To: Julien Grall <julien@xen.org>
Cc: Elliott Mitchell <ehem+undef@m5p.com>,
	xen-devel@lists.xenproject.org,
	Mitchell Horne <mhorne@freebsd.org>
Subject: Re: Uses of /hypervisor memory range (was: FreeBSD/Xen/ARM issues)
Date: Fri, 14 May 2021 12:07:53 +0200	[thread overview]
Message-ID: <YJ5L+ar29Q8g+xm+@Air-de-Roger> (raw)
In-Reply-To: <93936406-574f-7fd0-53bf-3bafaa4b1947@xen.org>

On Fri, May 14, 2021 at 09:32:10AM +0100, Julien Grall wrote:
> Hi Elliott,
> 
> On 14/05/2021 03:42, Elliott Mitchell wrote:
> > Was it intended for the /hypervisor range to dynamically scale with the
> > size of the domain?
> As per above, this doesn't depend on the size of the domain. Instead, this
> depends on what sort of the backend will be present in the domain.

It should instead scale based on the total memory on the system, ie:
if your hardware has 4GB of RAM the unpopulated range should at least
be: 4GB - memory of the current domain, so that it could map any
possible page assigned to a different domain (and even then I'm not
sure we shouldn't account for duplicated mappings).

> > Might it be better to deprecate the /hypervisor range and have domains
> > allocate any available address space for foreign mappings?
> 
> It may be easy for FreeBSD to find available address space but so far this
> has not been the case in Linux (I haven't checked the latest version
> though).
> 
> To be clear, an OS is free to not use the range provided in /hypervisor
> (maybe this is not clear enough in the spec?). This was mostly introduced to
> overcome some issues we saw in Linux when Xen on Arm was introduced.
> 
> > 
> > Should the FreeBSD implementation be treating grant tables as distinct
> > from other foreign mappings?
> 
> Both require unallocated addres space to work. IIRC FreeBSD is able to find
> unallocated space easily, so I would recommend to use it.

I agree. I think the main issue here is that there seems to be some
bug (or behavior not understood properly) with the resource manager
on Arm that returns an error when requesting a region anywhere in the
memory address space, ie: [0, ~0].

> > (is treating them the same likely to
> > induce buggy behavior on x86?)
> 
> I will leave this answer to Roger.

x86 is already treating them the same by using xenmem_alloc to request
memory to map the grant table or foreign mappings, so there's no
change on x86 in that regard.

Maybe I'm not getting that last question right.

Roger.