Re: [RFC PATCH] xen/memory: Introduce a hypercall to provide unallocated space

[Oleksandr <olekstysh@xxxxxxxxx>]

On 07.09.21 11:53, Henry Wang wrote:
> Hi Oleksandr,
Hi Henry


> > -----Original Message-----
> > From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
> > Oleksandr Tyshchenko
> > Sent: Thursday, July 29, 2021 12:19 AM
> > To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>; Daniel De
> > Graaf <dgdegra@xxxxxxxxxxxxx>; Daniel P. Smith
> > <dpsmith@xxxxxxxxxxxxxxxxxxxx>; Ian Jackson <iwj@xxxxxxxxxxxxxx>; Wei
> > Liu <wl@xxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; George
> > Dunlap <george.dunlap@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>;
> > Julien Grall <julien@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>;
> > Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; Roger Pau Monné
> > <roger.pau@xxxxxxxxxx>; Bertrand Marquis <Bertrand.Marquis@xxxxxxx>;
> > Wei Chen <Wei.Chen@xxxxxxx>
> > Subject: [RFC PATCH] xen/memory: Introduce a hypercall to provide
> > unallocated space
> >
> > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> >
> > Add XENMEM_get_unallocated_space hypercall which purpose is to
> > query hypervisor to find regions of guest physical address space
> > which are unused and can be used to create grant/foreign mappings
> > instead of wasting real pages from the domain memory for
> > establishing these mappings. The problem with the current Linux
> > on Xen on Arm behaviour is if we want to map some guest memory
> > regions in advance or to perform cache mappings in the backend
> > we might run out of memory in the host (see XSA-300).
> > This of course, depends on the both host and guest memory sizes.
> >
> > The "unallocated space" can't be figured out precisely by
> > the domain on Arm without hypervisor involvement:
> > - not all device I/O regions are known by the time domain starts
> >    creating grant/foreign mappings
> > - the Dom0 is not aware of memory regions used for the identity
> >    mappings needed for the PV drivers to work
> > In both cases we might end up re-using these regions by
> > a mistake. So, the hypervisor which maintains the P2M for the domain
> > is in the best position to provide "unallocated space".
> >
> > The arch code is in charge of finding these regions and filling
> > in corresponding array in new helper arch_get_unallocated_space().
> >
> > This patch implements common and Arm parts, the x86 specific bits
> > are left uniplemented for now.
> >
> > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> Tested-by: Henry Wang <Henry.Wang@xxxxxxx>
Thank you!


Please note, I have pushed new version (which is notably different) 
based on the discussion in current thread:
https://lore.kernel.org/xen-devel/1631034578-12598-1-git-send-email-olekstysh@xxxxxxxxx/



> Kind regards,
>
> Henry
--
Regards,

Oleksandr Tyshchenko