On Tue, 7 Apr 2020 12:36:34 +0100 Anthony PERARD <anthony.per...@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, >
