Re: [PATCH] libxl: fix "xl mem-set" regression from 0c029c4da2

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2015-04-22 at 15:41 +0100, Jan Beulich wrote:
> >>> On 22.04.15 at 16:01, <ian.campbell@citrix.com> wrote:
> > On Wed, 2015-04-22 at 13:02 +0100, Jan Beulich wrote:
> >> Said commit ("libxl_set_memory_target: retain the same maxmem offset on
> >> top of the current target") caused a regression for "xl mem-set"
> >> against Dom0: While prior to creation of the first domain this works,
> >> the first domain creation involving ballooning breaks. Due to "enforce"
> >> not being set in the domain creation case, and due to Dom0's initial
> >> ->max_pages (in the hypervisor) being UINT_MAX, the calculation of
> >> "memorykb" in the first "xl mem-set" adusting the target upwards
> >> subsequent to domain creation and termination may cause an overflow,
> >> resulting in Dom0's maximum getting to a very small value. This small
> >> maximum will the make the subsequent setting of the PoD target fail.
> >> 
> >> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> >> ---
> >> Note that this only fixes the immediate problem - there appear to be
> >> further issues lurking here:
> >> - libxl_set_memory_target()'s *_memkb variables all being 32-bit,
> >> - libxl_domain_setmaxmem()'s max_memkb parameter being 32-bit,
> > 
> > I think that increasing the width of these variables wouldn't break the
> > API guarantee which we make, at least not in a practical way, since any
> > existing 32-bit arguments passed will just get promoted.
> 
> No, not even on 64-bit. On 32-bit, two arguments slots are needed
> for what so far requires only one. On 64-bit (at least x86-64), the
> calling code isn't required to zero-extend a value calculated in a
> register (e.g. a result of earlier calculations which had more than
> 32 significant bits could be passed unchanged to the called function);
> it just so happens that 32-bit arithmetic on registers would always
> implicitly zero the upper halves (and iirc that's the same on ARM64).

You seem to be talking about ABI? As I tried to note in my response for
libxl we only make guarantees about the API (P not B in the middle).

Ian.