Re: [Qemu-devel] [PATCH v2 8/8] spapr_pci: Use XICS interrupt allocator and do not cache interrupts in PHB

[Alexey Kardashevskiy]

On 05/28/2014 09:35 PM, Alexander Graf wrote:
> 
> On 28.05.14 03:18, Alexey Kardashevskiy wrote:
>> On 05/28/2014 10:41 AM, Alexander Graf wrote:
>>> On 28.05.14 02:34, Alexey Kardashevskiy wrote:
>>>> On 05/28/2014 09:55 AM, Alexander Graf wrote:
> 
> ...
> 
>>>>> How do I migrate GHashTable? If I am allowed to use custom and bit more
>>>>> polished get/put from "[PATCH 1/2] vmstate: Add helper to enable
>>>>> GHashTable
>>>>> migration", I'll be fine.
>>>>> Yeah, I think it's ok to be custom in this case. Or another crazy idea -
>>>>> could you flatten the hash table into an array of structs that you can
>>>>> describe using VMState? You could then convert from the flat array
>>>>> to/from
>>>>> the GHashTable with pre_load/post_load hooks.
>>>> Array is exactly what I am trying to get rid of. Ok, I'll remove
>>>> hashmap at
>>>> all and implement dynamic flat array (yay, yet another bicycle!).
>>> Huh? The array would only live during the migration. It would be size=0
>>> during normal execution, but in a pre_save hook we could make size =
>>> hash.length() and reuse the existing, working VMState infrastructure.
>> When would I free that array? What if I continue the source guest and then
>> migrate again?
> 
> Something like
> 
> void pre_save(...) {
>     free(s->array);
>     s->array_len = s->hash.number_of_keys();
>     s->array = g_malloc(s->array_len * sizeof(struct array_elem));
>     for (i = 0; i < s->array_len; i++) {
>         s->array[i].key = s->hash.key[i];
>         s->array[i].value = s->hash.value[i];
>     }
> }
> 
> That would waste a few bytes when we continue after migration, but it
> should at least keep that overhead to a minimum.


Ok. Fine. When do I allocate an array on the destination then? Remember, I
do not know the number of device being transferred in advance because of
PCI hotplug so I cannot guess. sPAPRPHBState::pre_load is too early - I do
not know the size yet.

I can:
1. transfer size separately as part of sPAPRPHBState
2. move this temporary array into a subsection
3. allocate array in the subsection's pre_load() in a hope that QEMU will
call subsection's pre_load() AFTER the size of array is transferred.

This is true for now but what if one day someone decides that all
pre_load() callbacks from all subsections must be called at once at the
beginning of the object migration? I am screwed then.



Oooor I can make a patch as below (did not test it at all, just an idea).
Basically define VMS_ALLOC flag which will allocate necessary amount of
memory for that thing.


I am definitely missing somewhere here, as usual, and there must be already
some cool hack which I do not see, so what is it?



diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
index 6af599d..7a14d26 100644
--- a/include/migration/vmstate.h
+++ b/include/migration/vmstate.h
@@ -101,6 +101,7 @@ enum VMStateFlags {
     VMS_VARRAY_UINT8     = 0x400,  /* Array with size in uint8_t field*/
     VMS_VARRAY_UINT32    = 0x800,  /* Array with size in uint32_t field*/
     VMS_MUST_EXIST       = 0x1000, /* Field must exist in input */
+    VMS_ALLOC            = 0x2000, /* Alloc a buffer on the destination */
 };

 typedef struct {
@@ -750,6 +751,16 @@ static const VMStateInfo vmstate_info_hash;
     .offset     = vmstate_offset_value(_state, _field, qemu_hash),      \
 }

+#define VMSTATE_VARRAY_STRUCT_ALLOC(_field, _state, _field_num, _version,
_info, _type) {\
+    .name       = (stringify(_field)),                               \
+    .version_id = (_version),                                        \
+    .num_offset = vmstate_offset_value(_state, _field_num, int32_t), \
+    .info       = &(_info),                                          \
+    .size       = sizeof(_type),                                     \
+    .flags      = VMS_VARRAY_INT32|VMS_POINTER|VMS_ALLOC,            \
+    .offset     = vmstate_offset_pointer(_state, _field, _type),     \
+}
+
 #define VMSTATE_UNUSED_V(_v, _size)                                   \
     VMSTATE_UNUSED_BUFFER(NULL, _v, _size)

diff --git a/vmstate.c b/vmstate.c
index e1518da..7d6b0b9 100644
--- a/vmstate.c
+++ b/vmstate.c
@@ -48,6 +48,10 @@ static void *vmstate_base_addr(void *opaque,
VMStateField *field)
     void *base_addr = opaque + field->offset;

     if (field->flags & VMS_POINTER) {
+        if (field->flags & VMS_ALLOC) {
+            n_elems = vmstate_n_elems(opaque, field);
+            *base_addr = g_malloc_n(n_elems, field->size);
+        }
         base_addr = *(void **)base_addr + field->start;
     }



>> I mean I can solve all of this for sure but duplicating data
>> just to make existing migration happy is bit weird. But - I'll do what you
>> say here, it is no big deal :)
> 
> I don't find the concept of duplicating data for migration too odd - it
> sounds like a good compromise between introspectability and abstraction. If
> you have a better suggestion I'm all open :).




-- 
Alexey