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

[Juergen Gross <jgross@xxxxxxxx>]

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).


Juergen

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