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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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

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