Re: [PATCH] multifd: Copy pages before compressing them with zlib

[Dr. David Alan Gilbert]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On Mon, 4 Jul 2022 at 17:43, Ilya Leoshkevich <iii@linux.ibm.com> wrote:
> >
> > zlib_send_prepare() compresses pages of a running VM. zlib does not
> > make any thread-safety guarantees with respect to changing deflate()
> > input concurrently with deflate() [1].
> >
> > One can observe problems due to this with the IBM zEnterprise Data
> > Compression accelerator capable zlib [2]. When the hardware
> > acceleration is enabled, migration/multifd/tcp/plain/zlib test fails
> > intermittently [3] due to sliding window corruption. The accelerator's
> > architecture explicitly discourages concurrent accesses [4]:
> >
> >     Page 26-57, "Other Conditions":
> >
> >     As observed by this CPU, other CPUs, and channel
> >     programs, references to the parameter block, first,
> >     second, and third operands may be multiple-access
> >     references, accesses to these storage locations are
> >     not necessarily block-concurrent, and the sequence
> >     of these accesses or references is undefined.
> >
> > Mark Adler pointed out that vanilla zlib performs double fetches under
> > certain circumstances as well [5], therefore we need to copy data
> > before passing it to deflate().
> >
> > [1] https://zlib.net/manual.html
> > [2] https://github.com/madler/zlib/pull/410
> > [3] https://lists.nongnu.org/archive/html/qemu-devel/2022-03/msg03988.html
> > [4] http://publibfp.dhe.ibm.com/epubs/pdf/a227832c.pdf
> > [5] https://gitlab.com/qemu-project/qemu/-/issues/1099
> 
> Is this [5] the wrong link? It's to our issue tracker, not zlib's
> or a zlib mailing list thread, and it doesn't contain any messages
> from Mark Adler.

Looking at Mark's message, I'm not seeing that it was cc'd to the lists.
I did however ask him to update zlib's docs to describe the requirement.

Dave

> thanks
> -- PMM
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK