Re: [PATCH V3 (resend) 06/19] x86: Add a boot option to enable and disable the direct map

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.05.2024 11:19, Roger Pau Monné wrote:
> On Wed, May 15, 2024 at 03:54:51PM +0200, Jan Beulich wrote:
>> On 14.05.2024 11:20, Roger Pau Monné wrote:
>>> On Mon, May 13, 2024 at 01:40:33PM +0000, Elias El Yandouzi wrote:
>>>> --- a/docs/misc/xen-command-line.pandoc
>>>> +++ b/docs/misc/xen-command-line.pandoc
>>>> @@ -799,6 +799,18 @@ that enabling this option cannot guarantee anything 
>>>> beyond what underlying
>>>>  hardware guarantees (with, where available and known to Xen, respective
>>>>  tweaks applied).
>>>>  
>>>> +### directmap (x86)
>>>> +> `= <boolean>`
>>>> +
>>>> +> Default: `true`
>>>> +
>>>> +Enable or disable the directmap region in Xen.
>>>
>>> Enable or disable fully populating the directmap region in Xen.
>>
>> Elias, would you please take care to address earlier review comments
>> before sending a new version? This and ...
>>
>>>> +
>>>> +By default, Xen creates the directmap region which maps physical memory
>>>                                                           ^ all?
>>>> +in that region. Setting this to no will sparsely populate the directmap,
>>>
>>> "Setting this to no" => "Disabling this option will sparsely..."
>>
>> ... this is what I had already asked for in reply to v2, of course with
>> different wording.
>>
>>>> --- a/xen/arch/x86/setup.c
>>>> +++ b/xen/arch/x86/setup.c
>>>> @@ -1517,6 +1517,8 @@ void asmlinkage __init noreturn __start_xen(unsigned 
>>>> long mbi_p)
>>>>      if ( highmem_start )
>>>>          xenheap_max_mfn(PFN_DOWN(highmem_start - 1));
>>>>  
>>>> +    printk("Booting with directmap %s\n", has_directmap() ? "on" : "off");
>>>
>>> IMO this wants to be printed as part of the speculation mitigations, see
>>> print_details() in spec_ctrl.c
>>
>> And not "on" / "off", but "full" / "sparse" (and word order changed 
>> accordingly)
>> perhaps.
> 
> I've been thinking about this, and I'm leaning towards calling this
> new mode "ondemand" rather than "sparse".  The fact that the direct
> map ends up sparely populated is a consequence of populating it on
> demand, and hence the later would be more descriptive IMO.
> 
> (Same for the Kconfig option then ONDEMAND_DIRECTMAP, or some such)

Fine with me, fwiw.

Jan