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Re: [PATCH for-5.0] xen: fixup RAM memory region initialization
From: |
Igor Mammedov |
Subject: |
Re: [PATCH for-5.0] xen: fixup RAM memory region initialization |
Date: |
Wed, 8 Apr 2020 10:45:36 +0200 |
On Tue, 7 Apr 2020 12:36:34 +0100
Anthony PERARD <address@hidden> 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,
>