Re: [PATCH v3] xen: introduce XENFEAT_direct_mapped and XENFEAT_not_direct_mapped

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.03.2021 21:01, Stefano Stabellini wrote:
> On Mon, 15 Mar 2021, Jan Beulich wrote:
>> On 13.03.2021 00:16, Stefano Stabellini wrote:
>>> Introduce two feature flags to tell the domain whether it is
>>> direct-mapped or not. It allows the guest kernel to make informed
>>> decisions on things such as swiotlb-xen enablement.
>>>
>>> The introduction of both flags (XENFEAT_direct_mapped and
>>> XENFEAT_not_direct_mapped) allows the guest kernel to avoid any
>>> guesswork if one of the two is present, or fallback to the current
>>> checks if neither of them is present.
>>>
>>> XENFEAT_direct_mapped is always set for not auto-translated guests.
>>>
>>> For auto-translated guests, only Dom0 on ARM is direct-mapped. Also,
>>> see is_domain_direct_mapped() which refers to auto-translated guests:
>>> xen/include/asm-arm/domain.h:is_domain_direct_mapped
>>> xen/include/asm-x86/domain.h:is_domain_direct_mapped
>>>
>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
>>> CC: jbeulich@xxxxxxxx
>>> CC: andrew.cooper3@xxxxxxxxxx
>>> CC: julien@xxxxxxx
>>
>> Any particular reason my previously given R-b isn't here?
> 
> I reworded part of the comment in the public header, and I decided to
> err on the side of caution and not add your R-b given this change
> compared to the previous version.

I see. FAOD, despite me not being overly happy with the "older
Xen assumptions" part of the comment, feel free to add it back.

Jan