Re: Xen guest broken following "use memdev for RAM" patch

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 23 Mar 2020 17:10:39 +0000
Anthony PERARD <anthony.perard@citrix.com> wrote:

> Hi,
> 
> Since bd457782b3b0 ("x86/pc: use memdev for RAM") it isn't possible to
> start Xen guest anymore.
> 
> The error from qemu:
>     qemu-system-i386: xen: failed to populate ram at 0
> that comes from xen_ram_alloc() in hw/i386/xen/xen-hvm.c
> 
> xen_ram_alloc() is used to populate ram for PCI rom devices for example,
> but it is also called for the guest RAM. We try to detect when the call
> is for the ram by comparing the memory region with the one we created
> during initialisation.
> 
> During initialisation of QEMU  for a Xen guest, we create a memory
> region for the RAM by calling memory_region_init_ram() in
> xen_ram_init(). But that memory region isn't used by QEMU anymore (since
> mc->default_ram_id is set).
> 
> For Xen, we don't want QEMU to allocate the RAM, so using the memdev
> won't work.
> 
> Do you have a suggestion on
> - how we can bypass the generic code that allocate ram?
> - keep using something similar to what we have now with
>   memory_region_init_ram()?
> (with accel=xen)

Not that I like it but as a quick fix, does following work for you?

diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index e2d98243bc..bbca43bf33 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -953,6 +953,10 @@ static void xenfv_machine_options(MachineClass *m)
     m->desc = "Xen Fully-virtualized PC";
     m->max_cpus = HVM_MAX_VCPUS;
     m->default_machine_opts = "accel=xen";
+    /*
+     * opt out of system RAM being allocated by generic code
+     */
+    m->default_ram_id = NULL;
 }
 
 DEFINE_PC_MACHINE(xenfv, "xenfv", pc_xen_hvm_init,


> 
> Maybe we need a new hostmem backend which don't allocate any memory but
> creates a memory region, to be used by Xen?

Maybe, but I don't know about Xen enough so it's hard to make a suggestion,
so lets try to think out of the way to figure out how to approach it.

(In general, I'd prefer to get rid of dependency on 'accel' in memory_region_init_ram())

adapting xen code to use memdev isn't straightforward, since xen_ram_init()
fixes up memory_region size, it could be user provided
  ram_size
or
  4G + ram_size - HVM_BELOW_4G_RAM_END

question is why do you need to use memory_region_init_ram() at all if you are not actually
doing any allocation?
 
> Thanks,
>