Re: [Qemu-devel] [PATCH 4/9] qemu_qsb.diff

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 03/13/2013 05:11 PM, mdroth wrote:
> On Wed, Mar 13, 2013 at 01:56:23PM -0500, Joel Schopp wrote:
>> This patch adds support functions for operating on in memory sized file buffers.
> There's been some past refactorings to remove non-migration users of
> QEMUFile, and AFAIK that's still the case today. QEMUFile satisfies
> funky requirements like rate-limiting, buffering, etc that were specific
> to migration.
>
> IIUC all we want here is an abstraction on top of write()/memcpy(),
> and access to qemu_{put|get}_be* utility functions.
>
> Have you considered rolling those abstractions in the visitor
> implementations as opposed to extending QEMUFile, and using
> be*_to_cpus/cpus_to_be* helpers directly instead (like block/qcow2.c
> does, for example)?

The advantage of using the QEMUFile abstractions is that now you can 
build a visitor on top of it and read from buffers, sockets, BDRV's 
(later on), plain files, and whatever else you can hide underneath that 
interface. Back in 2011 when I initially wrote this code there at least 
was talk about using ASN.1 for migration, but this is nearly 2 years ago 
and it may never be done that way, so this was one driving force behind 
using QEMUFile inside the visitor. Besides that we later want to use the 
visitors for writing into virtual NVRAM, which we would build on top of 
a QEMUFile wrapping BDRVs. So there are some immediate advantages of 
using the common QEMUFile interface for reading and writing of data from 
different types of sources.

    Regards,
       Stefan