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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 16 Mar 2021, Jan Beulich wrote:
> 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.

Thank you!

Julien, please go ahead and commit it to your for-next/4.16 branch when
it is convenient.