Re: [Xen-devel] [PATCH v2 1/4] x86/boot: Remove the preconstructed low 16M superpage mappings

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.01.2020 20:32, Andrew Cooper wrote:
> These are left over from c/s b2804422 "x86: make Xen early boot code
> relocatable", which made it possible for Xen not to be in the bottom 16M.
> 
> Nothing using the mappings any more.  Build them in the directmap when walking
> the E820 table along with everything else.
> 
> Furthermore, it is undefined to have superpages and MTRRs disagree on
> cacheability boundaries, and nothing actually checks.  While we don't fix this
> explicitly, we do at least honour the E820 now if it says there are boundaries
> in this range.
> 
> As a consequence, there are now no _PAGE_PRESENT entries between
> __page_tables_{start,end} which need to skip relocation.  This simplifies the
> MB1/2 entry path logic to remove the l2_identmap[] special case.
> 
> The low 2M (using 4k pages) is retained for now.  Amongst other things, it
> matters for console logging while the legacy VGA hole is in use.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

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