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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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