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

[Juergen Gross <jgross@xxxxxxxx>]

On 07.04.22 16:44, Anthony PERARD wrote:
> 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().
Correct. And it refuses to do so when memory/static-max is below the
new memory size to be set.

> 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`.
When having e.g. a domain with 1G of maxmem, then calling

xl mem-max <domain> 2048

it should be possible to then do

xl mem-set <domain> 2048

but this isn't possible, as xl mem-set will look at the memory/static-max
node of the domain and refuse to do the setting if it has a too low
value, which is the case today.

> 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.
Oh, this should then be adapted. Today hotplugging memory is just
a mess as you need to:

xl mem-max <domid> <value>
xenstore-write /local/domain/<domid>/memory/static-max $((<value> * 1024))
xl mem-set <domid> <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.
Yes.

> >           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?
Normally it doesn't care at all.

> If yes, there's documentation and comments that needs to change with the
> code change.
I agree. Will do so.


Juergen

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