Re: Question to mem-path support at QEMU for Xen

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 28 Jul 2022 15:17:49 +0800
Huang Rui <ray.huang@amd.com> wrote:

> Hi Igor,
> 
> Appreciate you for the reply!
> 
> On Wed, Jul 27, 2022 at 04:19:30PM +0800, Igor Mammedov wrote:
> > On Tue, 26 Jul 2022 15:27:07 +0800
> > Huang Rui <ray.huang@amd.com> wrote:
> >   
> > > Hi Anthony and other Qemu/Xen guys,
> > > 
> > > We are trying to enable venus on Xen virtualization platform. And we would
> > > like to use the backend memory with memory-backend-memfd,id=mem1,size=4G
> > > options on QEMU, however, the QEMU will tell us the "-mem-path" is not
> > > supported with Xen. I verified the same function on KVM.
> > > 
> > > qemu-system-i386: -mem-path not supported with Xen
> > > 
> > > So may I know whether Xen has any limitation that support
> > > memory-backend-memfd in QEMU or just implementation is not done yet?  
> > 
> > Currently Xen doesn't use guest RAM allocation the way the rest of
> > accelerators do. (it has hacks in memory_region/ramblock API,
> > that divert RAM allocation calls to Xen specific API)  
> 
> I am new for Xen and QEMU, we are working on GPU. We would like to have a
> piece of backend memroy like video memory for VirtIO GPU to support guest
> VM Mesa Vulkan (Venus). Do you mean we can the memory_region/ramblock APIs
> to work around it?
> 
> > 
> > The sane way would extend Xen to accept RAM regions (whatever they are
> > ram or fd based) QEMU allocates instead of going its own way. This way
> > it could reuse all memory backends that QEMU provides for the rest of
> > the non-Xen world. (not to mention that we could drop non trivial
> > Xen hacks so that guest RAM handling would be consistent with other
> > accelerators)
> >   
> 
> May I know what do you mean by "going its own way"? This sounds good, could
> you please elaborate on how can we implement this? We would like to give a
> try to address the problem on Xen. Would you mind to point somewhere that I
> can learn and understand the RAM region. Very happy to see your
> suggestions!

see for example see ram_block_add(), if Xen could be persuaded to use memory
allocated by '!xen_enabled()' branch then it's likely file base backends
would also become usable.

Whether it is possible for Xen or not I don't know,
I guess CCed Xen folks will suggest something useful.
 
> Thanks & Best Regards,
> Ray
>