RE: [PATCH v2 1/7] exec: Fix for qemu_ram_resize() callback

[Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>]

> -----Original Message-----
> From: David Hildenbrand [mailto:david@redhat.com]
> Sent: 06 February 2020 10:56
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>;
> Igor Mammedov <imammedo@redhat.com>
> Cc: peter.maydell@linaro.org; xiaoguangrong.eric@gmail.com;
> mst@redhat.com; shannon.zhaosl@gmail.com; qemu-devel@nongnu.org;
> xuwei (O) <xuwei5@huawei.com>; Linuxarm <linuxarm@huawei.com>;
> eric.auger@redhat.com; qemu-arm@nongnu.org; lersek@redhat.com
> Subject: Re: [PATCH v2 1/7] exec: Fix for qemu_ram_resize() callback

[...]
 
> > root@ubuntu:/# cat /dev/pmem
> > pmem0  pmem1
> >
> > From the logs, it looks like the ram_load_precopy() --> qemu_ram_resize() is
> not
> > called as length == used_length and both seems to be page aligned values.
> > And from
> https://github.com/qemu/qemu/blob/master/migration/ram.c#L3421
> > qemu_ram_resize() is called with length if length != used_length.
> 
> Assume on your source, the old size is 12345 bytes. So 16384 aligned up
> (4 pages).
> 
> Assume on your target, the new size is 123456 bytes, so 126976 aligned
> up (31 pages).
> 
> If you migrate from source to destination, the migration code would
> resize to 16384, although the "actual size" is 12345. The callback will
> be called with the aligned size, not the actual size. Same the other way
> around. That's what's inconsistent IMHO.

Thanks. You are right. I didn’t consider the case where the target can be
configured with a larger number of devices than the source. I can replicate
the scenario now,

Source:

fw_cfg_add_file_callback: filename etc/boot-fail-wait size 0x4
fw_cfg_add_file_callback: filename etc/acpi/nvdimm-mem size 0x1000
fw_cfg_add_file_callback: filename etc/acpi/tables size 0x6210

Target:
ram_load_precopy: Ram blk mem1 length 0x40000000 used_length 0x40000000
ram_load_precopy: Ram blk virt.flash0 length 0x4000000 used_length 0x4000000
ram_load_precopy: Ram blk virt.flash1 length 0x4000000 used_length 0x4000000
ram_load_precopy: Ram blk /rom@etc/acpi/tables length 0x7000 used_length 0x8000
fw_cfg_modify_file: filename etc/acpi/tables len 0x7000

Target updates FWCfgEntry with a page aligned size :(. I will look into this and see how
we can solve this. Any pointers welcome.

Cheers,
Shameer