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Re: [Xen-devel] question about migration

On 25/12/2015 01:45, Wen Congyang wrote:
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:

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 

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.
I found another migration problem in the test:
If the migration fails, we will resume it in the source side.
But the hvm guest doesn't response any more.

In my test envirionment, the migration always successses, so I


use a hack way to reproduce it:
1. modify the target xen tools:

diff --git a/tools/libxl/libxl_stream_read.c b/tools/libxl/libxl_stream_read.c
index 258dec4..da95606 100644
--- a/tools/libxl/libxl_stream_read.c
+++ b/tools/libxl/libxl_stream_read.c
@@ -767,6 +767,8 @@ void libxl__xc_domain_restore_done(libxl__egc *egc, void 
          goto err;
+ rc = ERROR_FAIL;
      check_all_finished(egc, stream, rc);
2. xl cr hvm_nopv, and wait some time(You can login to the guest)
3. xl migrate hvm_nopv

The reason it that:
We create a default ioreq server when we get the hvm param HVM_PARAM_IOREQ_PFN.
It means that: the problem occurs only when the migration fails after we get
the hvm param HVM_PARAM_IOREQ_PFN.

In the function hvm_select_ioreq_server()
If the I/O will be handed by non-default ioreq server, we will return the
non-default ioreq server. In this case, it is handed by qemu.
If the I/O will not be handed by non-default ioreq server, we will return
the default ioreq server. Before migration, we return NULL, and after migration
it is not NULL.
See the caller is hvmemul_do_io():
         struct hvm_ioreq_server *s =
             hvm_select_ioreq_server(curr->domain, &p);

         /* If there is no suitable backing DM, just ignore accesses */
         if ( !s )
             rc = hvm_process_io_intercept(&null_handler, &p);
             vio->io_req.state = STATE_IOREQ_NONE;
             rc = hvm_send_ioreq(s, &p, 0);
             if ( rc != X86EMUL_RETRY || curr->domain->is_shutting_down )
                 vio->io_req.state = STATE_IOREQ_NONE;
             else if ( data_is_addr )
                 rc = X86EMUL_OKAY;

We send the I/O request to the default I/O request server, but no backing
DM hands it. We wil wait the I/O forever......

Hmm yes.  This needs fixing.

CC'ing Paul who did the ioreq server work.

This bug is caused by the read side effects of HVM_PARAM_IOREQ_PFN. The migration code needs a way of being able to query whether a default ioreq server exists, without creating one.

Can you remember what the justification for the read side effects were? ISTR that it was only for qemu compatibility until the ioreq server work got in upstream. If that was the case, can we drop the read side effects now and mandate that all qemus explicitly create their ioreq servers (even if this involves creating a default ioreq server for qemu-trad)?


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