Re: [Xen-devel] [PATCH v1] Fix p2m_set_suppress_ve

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.04.19 at 15:09, <tamas@xxxxxxxxxxxxx> wrote:
> I agree that it is confusing. It would be fine to UNSHARE here as well
> to keep things consistent but otherwise it's not really an issue as
> the entry type is checked later to ensure that this is a p2m_ram_rw
> entry. We are simply trying to keep mem_sharing and _modified_ altp2m
> entries exclusive. So it is fine to have mem_shared entries in the
> hostp2m and have those entries be copied into altp2m tables lazily,
> but for altp2m entries that have changed mem_access permissions or are
> remapped we want the entries in the hostp2m to be of regular type.
> This is not necessarily a technical requirement, it's mostly just to
> reduce complexity. So it would be fine to add UNSHARE here as well, I
> guess the only reason why I haven't done that is because I already
> trigger the unshare and copy-to-altp2m before remapping by setting
> dummy mem_access permission on the entries.

I'm afraid I don't agree with this justification: mem-sharing is about
contents of pages, whereas altp2m is about meta data (permissions
etc). I don't see why one would want to unshare because of a meta
data adjustment other than a page becoming non-CoW-writable.
Eagerly un-sharing in the end undermines the intentions of sharing.

Jan



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