Re: [Xen-devel] [PATCH v3 1/7] docs: Improve documentation for dom0= and dom0-iommu=

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/01/2019 09:14, Juergen Gross wrote:
> On 17/01/2019 10:08, Andrew Cooper wrote:
>> On 17/01/2019 08:43, Roger Pau Monné wrote:
>>> On Wed, Jan 16, 2019 at 07:51:33PM +0000, Andrew Cooper wrote:
>>>> On 16/01/2019 11:52, Jan Beulich wrote:
>>>>>>>> On 16.01.19 at 10:00, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>>> --- a/docs/misc/xen-command-line.pandoc
>>>>>> +++ b/docs/misc/xen-command-line.pandoc
>>>>>> @@ -636,61 +636,83 @@ trace feature is only enabled in debugging builds 
>>>>>> of Xen.
>>>>>>  
>>>>>>  Specify the bit width of the DMA heap.
>>>>>>  
>>>>>> -### dom0 (x86)
>>>>>> -> `= List of [ pvh | shadow | verbose ]`
>>>>>> +### dom0
>>>>>> +    = List of [ pvh=<bool>, shadow=<bool>, verbose=<bool> ]
>>>>>>  
>>>>>> -> Sub-options:
>>>>>> +    Applicability: x86
>>>>>>  
>>>>>> -> `pvh`
>>>>>> +Controls for how dom0 is constructed on x86 systems.
>>>>>>  
>>>>>> -> Default: `false`
>>>>>> +*   The `pvh` boolean controls whether dom0 is constructed as a PV or a 
>>>>>> PVH
>>>>>> +    guest.  The default is PV.  In addition, the following requirements 
>>>>>> must
>>>>>> +    be met:
>>>>>>  
>>>>>> -Flag that makes a dom0 boot in PVHv2 mode.
>>>>>> +    *   The dom0 kernel selected by the boot loader must be capable of 
>>>>>> the
>>>>>> +        selected mode.
>>>>>> +    *   For a PV dom0, Xen must have been compiled with `CONFIG_PV` 
>>>>>> enabled.
>>>>>> +    *   For a PVH dom0, Xen must have been compiled with `CONFIG_HVM` 
>>>>>> enabled,
>>>>>> +        and the hardware must have VT-x/SVM extensions available.
>>>>>>  
>>>>>> -> `shadow`
>>>>>> +*   The `shadow` boolean is only applicable when dom0 is constructed as 
>>>>>> a PVH
>>>>>> +    guest, and controls whether dom0 uses hardware assisted paging, or 
>>>>>> shadow
>>>>>> +    paging.  The default is HAP when available, and shadow otherwise.
>>>>>>  
>>>>>> -> Default: `false`
>>>>>> +    This option is unavailable when `CONFIG_SHADOW_PAGING` is compiled 
>>>>>> out.  A
>>>>>> +    PVH dom0 cannot be used if `CONFIG_SHADOW_PAGING` is compiled out, 
>>>>>> and the
>>>>>> +    hardware is not HAP-capable.
>>>>> As mentioned elsewhere, I object to adding CONFIG_* into this doc,
>>>>> which is intended to be meaningful to non-developers. But not to the
>>>>> degree of NAK-ing the whole thing, if everyone else disagrees with me.
>>>> I'm not sure what else to say.  I object to purposefully omitting
>>>> relevant information from our documentation.
>>> Maybe it would be helpful to add some kind of tag that could
>>> standardize the relationship between Kconfig options and command line
>>> options?
>>>
>>>     Kconfig: SHADOW_PAGING
>>>
>>> Or similar. This would get the specific Kconfig details out of the
>>> general description of the functionality, thus not harming readability
>>> by non-expert users?
>>>
>>> Using such tag would require some explanation of it's meaning at the
>>> top of the document.
>>>
>>>> Most people reading it, including non-developers, will know what Kconfig
>>>> is and how to check, owing to at least a basic knowledge of Linux. 
>>>> Those that don't will be capable of basic human interaction such as
>>>> asking a question of someone more knowledgeable.
>>> If the above is not suitable, I might suggest to reword the sentence
>>> as:
>>>
>>> "This option is unavailable when the Kconfig `SHADOW_PAGING` option is
>>> not selected at build time."
>>>
>>> Explicitly mentioning Kconfig and selected simplifies the language for
>>> non-expert users IMO, and makes it clear this is exclusively a build
>>> time decision. Note I'm not a native speaker, so my sense of easier to
>>> understand could be completely wrong.
>> I have a rewrite of the head of the document pending anyway which I hope
>> to get sorted properly for 4.12
>>
>> While having a Kconfig: section would probably be fine for ~80% of the
>> simple cases, it doesn't work for this patch.
>>
>> I guess the root of the issue is that I do not believe that phrasing the
>> information like this makes it harder for non-expert users
>> read/comprehend, and there definitely are a group of readers for which
>> this information is relevant.
> In any case I'd prefer to spell out the complete config option (e.g.
> CONFIG_FOO) in case we ever get a way to read the config from the
> running hypervisor (FWIW I'm just writing a series for doing that).

I think having a Xen equivalent of /proc/config.gz is a good move,
irrespective of any documentation concerns.

~Andrew

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