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Re: [Xen-devel] question about migration

On 12/24/2015 08:36 PM, Andrew Cooper wrote:
> 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
> 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.

Does FPU context have the similar problem?
IIRC, I have tested colo befroe 4.6 is released. It works. In my test, I always
use the option '-p' to start the HVM guest.

> 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 think it should work. But the user doesn't use the migration like this.
So it is not a serious problem.

Wen Congyang

> 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
> .

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