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Re: [Qemu-devel] Migration without memory page transfer
From: |
Peter Xu |
Subject: |
Re: [Qemu-devel] Migration without memory page transfer |
Date: |
Fri, 27 Apr 2018 10:45:11 +0800 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
On Thu, Apr 26, 2018 at 11:33:53PM +0000, Eric Wheeler wrote:
> Hello all,
Hi, Eric,
>
> This is my first time inside of the qemu code, so your help is greatly
> appreciated!
>
> I have been experimenting with stop/start of VMs to/from a migration
> stream that excludes RAM pages and let the RAM pages come from memory file
> provided by the memory-backend-file called '/dev/shm/mem'.
>
> To disable writing of memory pages to the migration stream, I've disabled
> calls to ram_find_and_save_block in ram_save_iterate() and
> ram_save_complete() (see patch below). Thus, the migration stream has the
> "ram" SaveStateEntry section start/ends, but no pages:
>
> qemu-system-x86_64 \
> -object
> memory-backend-file,prealloc=no,mem-path=/dev/shm/mem,id=ram-node0,host-nodes=0,policy=bind,size=64m,share=on
> \
> -numa node,nodeid=0,cpus=0,memdev=ram-node0\
> -m 64 -vnc 0:0
>
> Once the VM is running, I press ctrl-B to get the IPXE prompt and then
> run 'kernel http://192.168.0.1/foo' to start a network request and watch
> it in tcpdump.
>
> Once the download starts, I save the migration file:
> migrate "exec:cat > /dev/shm/t"
>
> # ls -lh /dev/shm/t
> -rw-r--r-- 1 root root 321K Apr 26 16:06 /dev/shm/t
>
> Now I can kill qemu and boot it again with -incoming:
>
> qemu-system-x86_64 \
> -object
> memory-backend-file,prealloc=no,mem-path=/dev/shm/mem,id=ram-node0,host-nodes=0,policy=bind,size=64m,share=on
> \
> -numa node,nodeid=0,cpus=0,memdev=ram-node0\
> -m 64 -vnc 0:0 \
> -incoming 'exec:cat /dev/shm/t'
>
> It seems to work. That is, network traffic continues (http from IPXE)
> which I can see from tcpdump. I can type into the console and it moves
> the cursor around---but there is nothing on the screen except the blinking
> text-mode cursor! I can even blindly start a new transfer in ipxe: kernel
> http://192.168.0.222/foo2 and see it in tcpdump.
>
> So what am I missing here? Is the video memory not saved to /dev/shm/mem?
>
> Or perhaps it is saved, but VGA isn't initialized to use what is
> already in /dev/shm/mem? I've tried the cirrus, std, and vmware drivers
> to see if they behave differently, but the do not seem to.
My wild guess is that we might still need to migrate some RAM besides
the /dev/shm/mem file. We have at least these ramblocks to migrate:
$ ./x86_64-softmmu/qemu-system-x86_64 -monitor stdio -m 2G
QEMU 2.12.0 monitor - type 'help' for more information
(qemu) info ramblock
Block Name PSize Offset Used
Total
pc.ram 4 KiB 0x0000000000000000 0x0000000080000000
0x0000000080000000
vga.vram 4 KiB 0x0000000080080000 0x0000000001000000
0x0000000001000000
/address@hidden/acpi/tables 4 KiB 0x0000000081100000 0x0000000000020000
0x0000000000200000
pc.bios 4 KiB 0x0000000080000000 0x0000000000040000
0x0000000000040000
0000:00:03.0/e1000.rom 4 KiB 0x00000000810c0000 0x0000000000040000
0x0000000000040000
pc.rom 4 KiB 0x0000000080040000 0x0000000000020000
0x0000000000020000
0000:00:02.0/vga.rom 4 KiB 0x0000000081080000 0x0000000000010000
0x0000000000010000
/address@hidden/table-loader 4 KiB 0x0000000081300000 0x0000000000001000
0x0000000000001000
/address@hidden/acpi/rsdp 4 KiB 0x0000000081340000 0x0000000000001000
0x0000000000001000
And my understanding is that /dev/shm/mem only corresponds to the
"pc.ram" entry. I suspect the rest of RAMBlocks will still need to be
migrated. For example, the VGA ram.
Meanwhile, could I ask about where will this be used? Is there
anything to do with something like a "distributed memory cache" that
provide memory service across multiple hosts?
Best Regards,
>
> Thanks for your help!
>
> --
> Eric Wheeler
>
>
> diff --git a/migration/ram.c b/migration/ram.c
> index 021d583..9f4bfff 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2267,9 +2267,9 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
> t0 = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
> i = 0;
> while ((ret = qemu_file_rate_limit(f)) == 0) {
> - int pages;
> + int pages = 0;
>
> - pages = ram_find_and_save_block(rs, false);
> + if (0) pages = ram_find_and_save_block(rs, false);
> /* no more pages to sent */
> if (pages == 0) {
> done = 1;
> @@ -2338,9 +2338,9 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
>
> /* flush all remaining blocks regardless of rate limiting */
> while (true) {
> - int pages;
> + int pages = 0;
>
> - pages = ram_find_and_save_block(rs, !migration_in_colo_state());
> + if (0) pages = ram_find_and_save_block(rs,
> !migration_in_colo_state());
> /* no more blocks to sent */
> if (pages == 0) {
> break;
>
>
--
Peter Xu