Re: [Qemu-devel] [PATCH v6 0/7] Rework vhost memory region updates

[Dr. David Alan Gilbert]

* Michael S. Tsirkin (address@hidden) wrote:
> On Thu, Jan 18, 2018 at 07:59:35PM +0000, Dr. David Alan Gilbert wrote:
> > * Michael S. Tsirkin (address@hidden) wrote:
> > > On Tue, Jan 16, 2018 at 06:04:01PM +0000, Dr. David Alan Gilbert (git) 
> > > wrote:
> > > > From: "Dr. David Alan Gilbert" <address@hidden>
> > > > 
> > > > Hi,
> > > >   This patch set reworks the way the vhost code handles changes in
> > > > physical address space layout that came from a discussion with Igor.
> > > > 
> > > > Its intention is to simplify a lot of the update code,
> > > > and to make it easier for the postcopy+shared code to
> > > > do the hugepage alignments that are needed.
> > > > 
> > > > Instead of inserting/removing each section during the add/del
> > > > callbacks of the listener, we start afresh and build a list
> > > > from the add and nop callbacks, then at the end compare the list
> > > > we've built with the exisiting list.
> > > > 
> > > > v6
> > > >   Tidy ups from Igor
> > > >   The biggest change is moving the 'Move log_dirty check' to be
> > > >   the last patch in the set.
> > > > 
> > > > Dr. David Alan Gilbert (7):
> > > >   vhost: Build temporary section list and deref after commit
> > > >   vhost: Simplify ring verification checks
> > > >   vhost: Merge sections added to temporary list
> > > >   vhost: Regenerate region list from changed sections list
> > > >   vhost: Clean out old vhost_set_memory and friends
> > > >   vhost: Merge and delete unused callbacks
> > > >   vhost: Move log_dirty check
> > > > 
> > > >  hw/virtio/trace-events    |   6 +
> > > >  hw/virtio/vhost.c         | 497 
> > > > ++++++++++++++++------------------------------
> > > >  include/hw/virtio/vhost.h |   5 +-
> > > >  3 files changed, 180 insertions(+), 328 deletions(-)
> > > 
> > > 
> > > Seems to trigger errors with clang runtime sanitizer:
> > > 
> > > /scm/qemu/hw/virtio/vhost.c:425:26: runtime error: null pointer passed as 
> > > argument 1, which is declared to never be null
> > > /usr/include/string.h:64:33: note: nonnull attribute specified here
> > > /scm/qemu/hw/virtio/vhost.c:425:45: runtime error: null pointer passed as 
> > > argument 2, which is declared to never be null
> > > /usr/include/string.h:64:33: note: nonnull attribute specified here
> > > /scm/qemu/hw/virtio/vhost.c:425:26: runtime error: null pointer passed as 
> > > argument 1, which is declared to never be null
> > > /usr/include/string.h:64:33: note: nonnull attribute specified here
> > > /scm/qemu/hw/virtio/vhost.c:425:45: runtime error: null pointer passed as 
> > > argument 2, which is declared to never be null
> > > /usr/include/string.h:64:33: note: nonnull attribute specified here
> > > /scm/qemu/hw/virtio/vhost.c:425:26: runtime error: null pointer passed as 
> > > argument 1, which is declared to never be null
> > > /usr/include/string.h:64:33: note: nonnull attribute specified here
> > > /scm/qemu/hw/virtio/vhost.c:425:45: runtime error: null pointer passed as 
> > > argument 2, which is declared to never be null
> > > /usr/include/string.h:64:33: note: nonnull attribute specified here
> > > /scm/qemu/hw/virtio/vhost.c:425:26: runtime error: null pointer passed as 
> > > argument 1, which is declared to never be null
> > > /usr/include/string.h:64:33: note: nonnull attribute specified here
> > > /scm/qemu/hw/virtio/vhost.c:425:45: runtime error: null pointer passed as 
> > > argument 2, which is declared to never be null
> > > /usr/include/string.h:64:33: note: nonnull attribute specified here
> > 
> > 
> > How are you running that test?
> 
> One Fedora:
> dnf install clang
> 
> '/scm/qemu-clang/../qemu/configure' '--cc=clang' '--cxx=clang++' 
> '--extra-cflags=-fsanitize=undefined -ferror-limit=10000 
> -fno-sanitize=shift-base'
> make
> make check.

Thanks.

> > Can you add this printf and tell me what
> > it's seeing?
> > 
> >         /* Same size, lets check the contents */
> >         fprintf(stderr, "%s: %p %p %d\n", __func__, dev->mem_sections, 
> > old_sections, n_old_sections);
> >         changed = memcmp(dev->mem_sections, old_sections,
> >                          n_old_sections * sizeof(old_sections[0])) != 0;
> > 
> > I'm seeing a bunch of calls where both pointers are NULL, but
> > n_old_sections is 0, which feels legal to me.
> 
> https://stackoverflow.com/questions/16362925/can-i-pass-a-null-pointer-to-memcmp
> 
> says it's invalid.

Posted v7 with trivial fix for it.
(It seems rather ungenerous of the standard to be fussy about that).

Dave

> > I guess we could make it:
> >    changed = n_old_sections ? memcmp(....) : false;
> > just to shut clang up.
> > 
> > Dave
> > 
> > > 
> > > 
> > > > -- 
> > > > 2.14.3
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK