[libvirt] [Xen-devel] [PATCH] libxl: fix dom0 autoballooning with Xen 4.8

[Wei Liu]

I saw this mail but didn't realise it required my input, sorry.

On Wed, Feb 01, 2017 at 04:00:54PM -0700, Jim Fehlig wrote:
> Jim Fehlig wrote:
> > xen.git commit 57f8b13c changed several of the libxl memory
> > get/set functions to take 64 bit parameters. The libvirt
> > libxl driver still uses uint32_t variables for these various
> > parameters, which is particularly problematic for the
> > libxl_set_memory_target() function.
> > 
> > When dom0 autoballooning is enabled, libvirt (like xl) determines
> > the memory needed to start a domain and the memory available. If
> > memory available is less than memory needed, dom0 is ballooned
> > down by passing a negative value to libxl_set_memory_target()
> > 'target_memkb' parameter. Prior to xen.git commit 57f8b13c,
> > 'target_memkb' was an int32_t. Subtracting a larger uint32 from
> > a smaller uint32 and assigning it to int32 resulted in a negative
> > number. After commit 57f8b13c, the same subtraction is widened
> > to a int64, resulting in a large positive number. The simple
> > fix taken by this patch is to assign the difference of the
> > uint32 values to a temporary int32 variable, which is then
> > passed to 'target_memkb' parameter of libxl_set_memory_target().
> > 
> > Note that it is undesirable to change libvirt to use 64 bit
> > variables since it requires setting LIBXL_API_VERSION to 0x040800.
> > Currently libvirt supports LIBXL_API_VERSION >= 0x040400,
> > essentially Xen >= 4.4.
> 
> Any comments on this patch? xen-devel is already in cc, but I'll add Wei (one of
> the Xen tools maintainers) and Juergen (author of commit 57f8b13c) for some
> additional exposure :-).
> 
> Currently, "out-of-the-box" Xen >= 4.8 + libvirt setup (dom0 autoballooning
> enabled) is broken without this fix.
> 
> Regards,
> Jim
> 
> > 
> > Signed-off-by: Jim Fehlig <jfehlig at suse.com>
> > ---
> >  src/libxl/libxl_domain.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/src/libxl/libxl_domain.c b/src/libxl/libxl_domain.c
> > index a5314b0..ed73cd2 100644
> > --- a/src/libxl/libxl_domain.c
> > +++ b/src/libxl/libxl_domain.c
> > @@ -909,6 +909,7 @@ libxlDomainFreeMem(libxl_ctx *ctx, libxl_domain_config *d_config)
> >  {
> >      uint32_t needed_mem;
> >      uint32_t free_mem;
> > +    int32_t target_mem;
> >      int tries = 3;
> >      int wait_secs = 10;
> >  
> > @@ -922,7 +923,8 @@ libxlDomainFreeMem(libxl_ctx *ctx, libxl_domain_config *d_config)
> >          if (free_mem >= needed_mem)
> >              return 0;
> >  
> > -        if (libxl_set_memory_target(ctx, 0, free_mem - needed_mem,
> > +        target_mem = free_mem - needed_mem;
> > +        if (libxl_set_memory_target(ctx, 0, target_mem,

This should do the trick.

Wei.

> >                                      /* relative */ 1, 0) < 0)
> >              goto error;
> >  
>