Re: [PATCH] libxl: fix dom0 autoballooning with Xen 4.8

[Jim Fehlig <jfehlig@suse.com>]

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


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