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

[Huang Rui]

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!

Thanks & Best Regards,
Ray