Re: [Xen-devel] [PATCH for-4.8] docs: document ACPI usage in PVHv2 guests

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 03/11/16 15:55, Jan Beulich wrote:
>>>> On 03.11.16 at 16:38, <roger.pau@xxxxxxxxxx> wrote:
>> On Thu, Nov 03, 2016 at 07:06:14AM -0600, Jan Beulich wrote:
>>>>>> On 03.11.16 at 13:19, <roger.pau@xxxxxxxxxx> wrote:
>>>> --- a/docs/misc/hvmlite.markdown
>>>> +++ b/docs/misc/hvmlite.markdown
>>>> @@ -64,3 +64,14 @@ The following VCPU hypercalls can be used in order to 
>> bring up secondary vCPUs:
>>>>   * `VCPUOP_down` is used to bring down a vCPU.
>>>>  
>>>>   * `VCPUOP_is_up` is used to scan the number of available vCPUs.
>>>> +
>>>> +## Hardware description ##
>>>> +
>>>> +PVHv2 guests that have access to hardware (either emulated or real) will 
>>>> also
>>>> +have ACPI tables with the description of the hardware that's available to 
>>>> the
>>>> +guest. This applies to both privileged and unprivileged guests. A hint of
>>>> +the position of the RSDP in memory (if present) can be fetched from the 
>>>> start
>>>> +info structure that's passed at boot time (field rsdp_paddr).
>>>
>>> Why "hint"?
>>
>> Hm, maybe that's not the best word? (my English is clearly lacking 
>> sometimes). What I meant is that this is a pointer to the RSDP. I've 
>> probably used "hint" here because in FreeBSD the position of the RSDP is 
>> passed from the loader to the kernel as "hint.acpi.0.rsdp". Let me know if 
>> you would like me to rephrase this.
> 
> I indeed think this should be re-worded. After all that start info field
> is iirc the _only_ way to get the pointer.

Yes, a hint, particularly in computer science, is a bit of information
which 1) isn't strictly needed, and 2) may not actually be correct; a
hint may be ignored with no change in functionality (only performance).
You might hint to the OS that a bit of memory is going to be very hot
and should therefore not be paged out; but the OS can ignore that hint
if it wants.  SCHEDOP_yield in Xen is a hint.

Since as Jan says, the information *is* strictly needed, and must always
be correct, you should probably just call it a pointer.

 -George

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