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

[Jim Fehlig]

On 02/02/2017 04:42 AM, Wei Liu wrote:
> I saw this mail but didn't realise it required my input, sorry.

I suppose it didn't and I was shamelessly fishing for a review - so you have my 
apologies :-). But the patch does fix an annoying, easily encountered bug which 
I'm eager to see resolved in the next libvirt release.

>
> 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.

Thanks. There are other ways to skin this cat and if folks prefer one of those I 
can push a followup. But for now, so that we have a fix, I'll push this variant.

Regards,
Jim