Re: [Xen-devel] [PATCH 2/2] domain: use PGC_extra domheap page for shared_info

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.03.2020 12:52, David Woodhouse wrote:
> On Fri, 2020-03-06 at 12:37 +0100, Jan Beulich wrote:
>> I've started looking at the latest version of Paul's series, but I'm
>> still struggling to see the picture: There's no true distinction
>> between Xen heap and domain heap on x86-64 (except on very large
>> systems). Therefore it is unclear to me what "those pages" is actually
>> referring to above. Surely new Xen can't be given any pages in use
>> _in any way_ by old Xen, no matter whether it's ones assigned to
>> domains, or ones used internally to (old) Xen.
> 
> Old and new Xen do not coexist. There is a kexec (via kexec_reloc.S and
> purgatory) from old to new.
> 
> There are some pages which new Xen MUST NOT scribble on, because they
> actually belong to the domains being preserved. That includes the EPT
> (or at least IOMMU) page tables.

And likely interrupt remapping tables, device tables, etc. I don't
have a clear picture on how you want to delineate ones in use in any
such way from ones indeed free to re-use.

Jan

> I suppose new Xen also mustn't scribble on the pages in which old Xen
> has placed the migration information for those domains either. At
> least, not until it's consumed the data.
> 
> Anything else, however, is fine for new Xen to scribble on. Fairly much
> anything that the old Xen had allocated from its xenheap (and not
> subsequently shared to a guest, qv) is no longer required and can be
> treated as free memory by the new Xen, which now owns the machine.
> 
> 


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel