Re: [PATCH 11/22] x86: add a boot option to enable and disable the direct map

[Elias El Yandouzi <eliasely@xxxxxxxxxx>]

Hi,

On 12/01/2024 07:47, Jan Beulich wrote:
> On 11.01.2024 19:25, Elias El Yandouzi wrote:
> > On 11/01/2024 14:09, Jan Beulich wrote:
> >
> > What about maddr_to_virt()? For instance, in the function
> > xen/arch/x86/dmi_scan.c:dmi_iterate(), we need to access a very low
> > machine address which isn't in the directmap range.
> I'm afraid I don't follow: Very low addresses are always in the
> direct map range, which - on x86 - always starts at 0.
I reckon it was poorly phrased. IIUC, we'd like to remove every use of 
*_to_virt() in the case the directmap option is disabled.
So I meant that in this situation, the helper arch_mfns_in_direct_map() 
would return false.
> > How would you proceed? Calling vmap() seems to be a bit overkill for
> > just a temporary mapping and I don't really want to rework this function
> > to use map_domain_page().
> >
> > In such case, how would you proceed? What do you suggest?
> fixmap may be an option to consider, but I also don't see why you
> apparently think using vmap() would be a possibility while at the
> same time making use of map_domain_page() is too much effort.
I thought about using vmap() as it allows to map a contiguous region 
easily. It is also used in the follow-up patch 17/22, so I thought it 
could be viable.
I was reluctant to use map_domain_page() for two reasons. 1) it only 
allows to map one page at the time, so I'd need to rework more deeply 
the function dmi_iterate() 2) because the mapcache wouldn't be ready to 
use at that time, the mapping would end up in PMAP which is meant to map 
the page tables, nothing else.
--
Elias