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 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?

> > * in X86 arch_monitor_domctl_event,
> > XEN_DOMCTL_MONITOR_EVENT_WRITE_CTRLREG case
> > add a sanity check of mop->u.mov_to_cr.index before:
> >       unsigned int ctrlreg_bitmask =
> > monitor_ctrlreg_bitmask(mop->u.mov_to_cr.index);
> > , which basically translates to:
> >       unsigned int ctrlreg_bitmask = (1U << mop->u.mov_to_cr.index);
> >
> > ? (especially since mop->u.mov_to_cr.index is set by the caller).
> Yes, there a range check would be needed, but preferably as a
> separate patch (as this has nothing to do with the code motion
> you perform here).
>
> Jan
Great, I'll do these changes in a separate patch then.
Let me know if you have any other comments on this.

Thanks,
Corneliu.

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