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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.04.19 at 16:43, <tamas@xxxxxxxxxxxxx> wrote:
> On Thu, Apr 4, 2019 at 8:36 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>
>> >>> 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,
> 
> That's incorrect. Mem sharing doesn't care about the contents of pages at 
> all.

Would you mind clarifying this? It's about sharing of the contents
of the pages, isn't it? (Of course the me-sharing code doesn't care
what the contents are.)

> 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.
> 
> We are unsharing to keep altp2m and mem_sharing compatible but
> mutually exclusive. Even if technically they could co-exist, last time
> I worked on this we came to this agreement on the mailinglist as to
> reduce complexity and to make reviewing easier.

But "mutually exclusive" to me means you can do only one of the
two on a particular VM. Which then makes it even less necessary
to request un-share from altp2m code - such requests would then
simple be no-ops.

Jan



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