Re: [Xen-devel] [PATCH v4 2/2] xen/vm-events: Move parts of monitor_domctl code to common-side.

[Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>]

On 2/16/2016 2:34 PM, Jan Beulich wrote:
> > > > On 16.02.16 at 12:20, <czuzu@xxxxxxxxxxxxxxx> wrote:
> > On 2/16/2016 12:45 PM, Jan Beulich wrote:
> > > > > > On 16.02.16 at 09:13, <czuzu@xxxxxxxxxxxxxxx> wrote:
> > > > On 2/16/2016 9:08 AM, Corneliu ZUZU wrote:
> > > > > This patch moves monitor_domctl to common-side.
> > > > > Purpose: move what's common to common, prepare for implementation
> > > > > of such vm-events on ARM.
> > > > >
> > > > > * move get_capabilities to arch-side => arch_monitor_get_capabilities.
> > > > > * add arch-side monitor op handling function => arch_monitor_domctl_op.
> > > > >      e.g. X86-side handles XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP op
> > > > > * add arch-side monitor event handling function => arch_monitor_domctl_event.
> > > > >      e.g. X86-side handles XEN_DOMCTL_MONITOR_EVENT_MOV_TO_MSR event
> > > > enable/disable
> > > > > * remove status_check
> > > > >
> > > > > Signed-off-by: Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>
> > > > >
> > > > > ---
> > > > > Changed since v3:
> > > > >      * monitor_domctl @ common/monitor.c:
> > > > >        - remove unused requested_status
> > > > >        - sanity check mop->event range to avoid left-shift undefined behavior
> > > > Due to left-shift undefined behavior situations, shouldn't I also:
> > > >
> > > > * in X86 arch_monitor_get_capabilities: replace '1 <<' w/ '1U <<'
> > > There's no undefinedness there, since the right side operands of
> > > << are all constant. Using 1U here would be okay, but is not
> > > strictly needed.
> > I reasoned based on this ISO C99 quote:
> > [for an E1 << E2 operation, ]
> > "If E1 has a signed type and nonnegative value, and E1 Ã 2^E2 is
> > representable in the result type, then that is the resulting value;
> > otherwise, the behavior is undefined."
> >
> > I inferred that this means that code such as '(1 << 31)' would render
> > undefined behavior, since (1 x 2^31) is not representable on 'int'.
> > The standard doesn't seem to mention different behavior if the operands
> > are constants.
> > This would render undefined behavior if bit 31 of capabilities would be
> > used @ some point, i.e. if one day someone would e.g. unknowingly:
> >       #define XEN_DOMCTL_MONITOR_EVENT_GRAVITATIONAL_WAVE 31
> > Have I misinterpreted the 'representable in the result type' part?
> No, that's all correct. It's just that right now no
> XEN_DOMCTL_MONITOR_EVENT_* has value 31, and hence
> there's only a very minor latent issue here (someone blindly
> copying the existing 1 << ... without adding the necessary U at
> that point; one might hope the compiler would then point this out
> though).
>
> Jan
Ah, I see. Did a fast test earlier w/ GCC 5.1, unfortunately I think the 
compiler doesn't
issue any warning in this situation, would have preferred a heads-up too 
(couldn't even force it to do so).
(it would be nice if Xen shipped w/ a gravitational-waves detector 
though....)
Corneliu.

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