Re: [PATCH for-5.0] xen: fixup RAM memory region initialization

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 7 Apr 2020 12:36:34 +0100
Anthony PERARD <anthony.perard@citrix.com> wrote:

> On Thu, Apr 02, 2020 at 04:30:33PM +0200, Igor Mammedov wrote:
> > 1. why xen uses memory_region_init_ram() which does not allocate anything  
> 
> This seems to be due to history.
> 
> >    can it use plain memory_region_init() instead?  
> 
> I can give it a try. And it doesn't work, I had to call qemu_ram_alloc() as
> well, to set ram_memory.c.
> 
> On the other and, I think memory_region_init_ram_nomigrate() would be
> enough. QEMU didn't complain when I migrated the guest.

Why does it need ramblock fir main ram if it does not allocate nor migrate
it using QEMU infrastructure?

> 
> > 2. how main ram is allocated?  
> 
> It's done by the toolstack, libxl. It creates a new domain in the
> hypervisor, memory allocation is part of this, then QEMU is started, for
> emulation of some devices.
> 
> There is one thing that QEMU does in regards to memory, it's the call
> xc_domain_populate_physmap_exact() in xen_ram_alloc(). This is for when
> an emulated PCI device needs some memory, like for the VGA region.
> 
> > 3. code has
> >            /*                                                                       
> >          * Xen does not allocate the memory continuously, it keeps a             
> >          * hole of the size computed above or passed in.                         
> >          */                                                                      
> >         block_len = (1ULL << 32) + x86ms->above_4g_mem_size; 
> >    which fixes up size ram memory region
> >    can we allocate 1 memory region of ram_size and then
> >    alias lower and upper memory instead of that?  
> 
> I don't know, I don't think I know enough about how memory_region are
> used to be able to answer that.
> 
> > 4. how RAM migration works in case of xen?  
> 
> From QEMU, we only migrate devices states, we call the
> "xen-save-devices-state" QMP command. Memory migration is done by the
> toolstack. In QEMU, there is a bodge in xen_ram_alloc() to avoid having
> QEMU doing some "allocation" during migration.
> 
> I hope that help.
> Cheers,
>