Re: question about migration

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 24/12/15 02:29, Wen Congyang wrote:
> Hi Andrew Cooper:
>
> I rebase the COLO codes to the newest upstream xen, and test it. I found
> a problem in the test, and I can reproduce this problem via the migration.
>
> How to reproduce:
> 1. xl cr -p hvm_nopv
> 2. xl migrate hvm_nopv 192.168.3.1

You are the very first person to try a usecase like this.

It works as much as it does because of your changes to the uncooperative 
HVM domain logic.  I have said repeatedly during review, this is not 
necessarily a safe change to make without an in-depth analysis of the 
knock-on effects; it looks as if you have found the first knock-on effect.

>
> The migration successes, but the vm doesn't run in the target machine.
> You can get the reason from 'xl dmesg':
> (XEN) HVM2 restore: VMCE_VCPU 1
> (XEN) HVM2 restore: TSC_ADJUST 0
> (XEN) HVM2 restore: TSC_ADJUST 1
> (d2) HVM Loader
> (d2) Detected Xen v4.7-unstable
> (d2) Get guest memory maps[128] failed. (-38)
> (d2) *** HVMLoader bug at e820.c:39
> (d2) *** HVMLoader crashed.
>
> The reason is that:
> We don't call xc_domain_set_memory_map() in the target machine.
> When we create a hvm domain:
> libxl__domain_build()
>      libxl__build_hvm()
>          libxl__arch_domain_construct_memmap()
>              xc_domain_set_memory_map()
>
> Should we migrate the guest memory from source machine to target machine?

This bug specifically is because HVMLoader is expected to have run and 
turned the hypercall information in an E820 table in the guest before a 
migration occurs.

Unfortunately, the current codebase is riddled with such assumption and 
expectations (e.g. the HVM save code assumed that FPU context is valid 
when it is saving register state) which is a direct side effect of how 
it was developed.


Having said all of the above, I agree that your example is a usecase 
which should work.  It is the ultimate test of whether the migration 
stream contains enough information to faithfully reproduce the domain on 
the far side.  Clearly at the moment, this is not the case.

I have an upcoming project to work on the domain memory layout logic, 
because it is unsuitable for a number of XenServer usecases. Part of 
that will require moving it in the migration stream.

~Andrew