Re: [Xen-devel] [PATCH] ARM: cache coherence problem in guestcopy.c

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-06-18 at 12:05 +0000, Jaeyong Yoo wrote:
> > We were also speculating that we probably want some DMBs in
> > context_switch_{from,to} as well as at return_to_guest.
> 
> Actually, I just learned ReOrder Buffer, and it looks like so.

Right. Tim and I were speculating about where and why the barriers were
needed:

In unmap_domain_page: This ensures any writes to domain memory via the
Xen mapping are seen by subsequent accesses via the guest mapping.
Likewise map_domain_page needs one to ensure that previous accesses via
the guest mapping will be observed (this is the issue you observed).

In context_switch_from: To ensure any writes done on a VCPU are seen
before it is rescheduled, potentially on another PCPU. It seems likely
that you'd want one in context_switch_to for the complementary reasons,

In return_to_guest: This is similar to the unmap_domain_page one but
handles writes to pages which are permanently shared by Xen and the
guest, e.g. the shared info page or anything mapped with
share_xen_page_with_guest etc. This one is debatable because all users
of this are, I think, expected to get their own barriers correct since
this these pages are typically part of some lock free datastructure
arrangement.

Ian.


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