Re: [PATCH] tools/libs/light: update xenstore entry when setting max domain memory

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Thu, Mar 31, 2022 at 09:07:55AM +0200, Juergen Gross wrote:
> libxl_domain_setmaxmem() should update the domain's memory/static-max
> Xenstore node, as otherwise "xl mem-set" won't be able to set the
> memory size to the new maximum.

`xl mem-set` doesn't call libxl_domain_setmaxmem(), but calls
libxl_set_memory_target().

Or maybe you are speaking about `xl mem-max` followed by `xl mem-set`?
In this case, it is documented in `man 1 xl` that `mem-max` has no
effect to `mem-set`.

quote from man, about `xl mem-max`:
    It is allowed to be higher than the configured maximum
    memory size of the domain (B<maxmem> parameter in the domain's
    configuration). Note however that the initial B<maxmem> value is still
    used as an upper limit for B<xl mem-set>.  Also note that calling B<xl
    mem-set> will reset this value.

> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  tools/libs/light/libxl_mem.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/tools/libs/light/libxl_mem.c b/tools/libs/light/libxl_mem.c
> index c739d00f39..2f4f9d4a4a 100644
> --- a/tools/libs/light/libxl_mem.c
> +++ b/tools/libs/light/libxl_mem.c
> @@ -82,6 +82,15 @@ int libxl_domain_setmaxmem(libxl_ctx *ctx, uint32_t domid, 
> uint64_t max_memkb)

There's a comment on this functions:
    /*
     * Set the maximum memory size of the domain in the hypervisor. There is no
     * change of the current memory size involved. The specified memory size can
     * even be above the configured maxmem size of the domain, but the related
     * Xenstore entry memory/static-max isn't modified!
     */
    int libxl_domain_setmaxmem(libxl_ctx *ctx, uint32_t domid, uint64_t 
max_memkb)

So it was already known that "static-max" wasn't set.
At the very least, this comment needs updating.

>          goto out;
>      }
>  
> +    rc = libxl__xs_printf(gc, XBT_NULL,
> +                          GCSPRINTF("%s/memory/static-max", dompath),
> +                          "%"PRIu64, max_memkb);
> +    if (rc != 0) {
> +        LOGED(ERROR, domid, "Couldn't set %s/memory/static-max, rc=%d\n",
> +              dompath, rc);
> +        goto out;



So, I don't know whether increasing "static-max" is fine or not, but
according to the documentation, it isn't expected.

Is a guest fine with "static-max" been changed?

If yes, there's documentation and comments that needs to change with the
code change.

Thanks,

-- 
Anthony PERARD