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Re: [Qemu-devel] [PATCH 1/3] vmstate: introduce calc_size VMStateField

From: Mitsyanko Igor
Subject: Re: [Qemu-devel] [PATCH 1/3] vmstate: introduce calc_size VMStateField
Date: Tue, 27 Dec 2011 12:11:38 +0400
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20111109 Thunderbird/3.1.16

On 12/26/2011 07:20 PM, Peter Maydell wrote:
On 26 December 2011 10:03, Mitsyanko Igor<address@hidden>  wrote:
New calc_size field in VMStateField is supposed to help us easily add 
support of dynamically allocated buffers in device's states.
There are some cases when information on size of dynamically allocated buffer is
already presented in specific device's state structure, but in such a form, that
can not be used with existing VMStateField interface. Currently, we either can 
get size from
another variable in device's state as it is with VMSTATE_VBUFFER_* macros, or 
we can
also multiply value kept in a variable by a constant with 
macro. If we need to perform any other action, we're forced to add additional
variable with size information to device state structure with the only intention
to use it in VMStateDescription structure. This approach is not very pretty. 
Adding extra
flags to VMStateFlags enum for every other possible operation with size field
seems redundant, and still it would't cover cases when we need to perform a set 
operations to get size value.
With this new .calc_size field we can calculate size of dynamic array in 
way we need.

Signed-off-by: Mitsyanko Igor<address@hidden>

It seems a bit curious that this patch removes the existing (although admittedly
unused) VMSTATE_BUFFER_MULTIPLY but doesn't actually say so in the
commit message.

  hw/hw.h  |   14 +++++++-------
  savevm.c |   14 ++++++++------
  2 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/hw/hw.h b/hw/hw.h
index efa04d1..8ce4475 100644
--- a/hw/hw.h
+++ b/hw/hw.h
@@ -303,9 +303,9 @@ enum VMStateFlags {
     VMS_ARRAY_OF_POINTER = 0x040,
     VMS_VARRAY_UINT16    = 0x080,  /* Array with size in uint16_t field */
     VMS_VBUFFER          = 0x100,  /* Buffer with size in int32_t field */
-    VMS_MULTIPLY         = 0x200,  /* multiply "size" field by field_size */
-    VMS_VARRAY_UINT8     = 0x400,  /* Array with size in uint8_t field*/
-    VMS_VARRAY_UINT32    = 0x800,  /* Array with size in uint32_t field*/
+    VMS_CALC_SIZE        = 0x200,  /* calculate size of dynamic buffer */
+    VMS_VARRAY_UINT8     = 0x400,  /* Array with size in uint8_t field */
+    VMS_VARRAY_UINT32    = 0x800,  /* Array with size in uint32_t field */

These unrelated whitespace fixes are confusing -- please drop them.
QEMU wiki here http://wiki.qemu.org/Contribute/SubmitAPatch states that it's ok :) > It's OK to fix coding style issues in the immediate area (few lines) > of the lines you're changing.)

Given some thoughts to it, I didn't like that this patch increases memory consumed by VMStateField. Maybe we should drop existing .size_offset field and replace it with generic get_buffsize callback, like we did in second version of this patch. It seems reasonable to me, even though existing .size_offset usage is pretty neat.

Mitsyanko Igor
ASWG, Moscow R&D center, Samsung Electronics
email: address@hidden

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