Re: [Qemu-devel] [PATCH v2] migration: avoid filling ignore-shared ramblock when in incoming migration

[Peter Xu <peterx@redhat.com>]

On Wed, Apr 03, 2019 at 11:21:47PM +0800, Catherine Ho wrote:
> Hi Peter Xu
> 
> On Wed, 3 Apr 2019 at 10:25, Peter Xu <peterx@redhat.com> wrote:
> 
> > On Tue, Apr 02, 2019 at 11:30:01AM -0400, Catherine Ho wrote:
> > > Commit 18269069c310 ("migration: Introduce ignore-shared capability")
> > > addes ignore-shared capability to bypass the shared ramblock (e,g,
> > > membackend + numa node). It does good to live migration.
> > >
> > > This commit expectes that QEMU doesn't write to guest RAM until
> > > VM starts, but it does on aarch64 qemu:
> > > Backtrace:
> > > 1  0x000055f4a296dd84 in address_space_write_rom_internal () at
> > exec.c:3458
> > > 2  0x000055f4a296de3a in address_space_write_rom () at exec.c:3479
> > > 3  0x000055f4a2d519ff in rom_reset () at hw/core/loader.c:1101
> > > 4  0x000055f4a2d475ec in qemu_devices_reset () at hw/core/reset.c:69
> > > 5  0x000055f4a2c90a28 in qemu_system_reset () at vl.c:1675
> > > 6  0x000055f4a2c9851d in main () at vl.c:4552
> > >
> > > Actually, on arm64 virt marchine, ramblock "dtb" will be filled into ram
> > > during rom_reset. In ignore-shared incoming case, this rom filling
> > > is not required since all the data has been stored in memory backend
> > file.
> > >
> > > Fixes: commit 18269069c310 ("migration: Introduce ignore-shared
> > capability")
> > > Signed-off-by: Catherine Ho <catherine.hecx@gmail.com>
> > > Suggested-by: Yury Kotov <yury-kotov@yandex-team.ru>
> >
> > (note: IIUC normally you should have your signed-off to be the last
> >  line before the suggested-by :)
> >
> > About the patch content, I have had a question on whether we should
> > need to check ignore-shared at all... That question lies in:
> >
> > https://patchwork.kernel.org/patch/10859889/#22546487
> >
> > And if my understanding was correct above, IMHO the patch could be as
> > simply be as "if (runstate_check(RUN_STATE_INMIGRATE)) return;" at [1]
> > below.
> >
> >
> Thanks， but I thought this method would break the x86 rom_reset logic during
> RUN_STATE_INMIGRATE.
> Please see the debugging patch and log lines below:
> diff --git a/hw/core/loader.c b/hw/core/loader.c
> index fe5cb24122..b0c871af26 100644
> --- a/hw/core/loader.c
> +++ b/hw/core/loader.c
> @@ -1086,8 +1086,9 @@ int rom_add_option(const char *file, int32_t
> bootindex)
>  static void rom_reset(void *unused)
>  {
>      Rom *rom;
> -
>      QTAILQ_FOREACH(rom, &roms, next) {
> +        if (runstate_check(RUN_STATE_INMIGRATE))
> +           printf("rom name=%s\n",rom->name);
>          if (rom->fw_file) {
>              continue;
>          }
> 
> rom name=kvmvapic.bin
> rom name=linuxboot_dma.bin
> rom name=bios-256k.bin
> rom name=etc/acpi/tables
> rom name=etc/table-loader
> rom name=etc/acpi/rsdp

Hi, Catherine,

I only see that rom names were dumped.  Could you help explain what is
broken?  Thanks,

-- 
Peter Xu