Re: memaccess: skipping mem_access_send_req

[Ian Campbell <ian.campbell@citrix.com>]

On Fri, 2015-04-17 at 10:35 +0100, Gareth Stockwell wrote:
> On Wed, Apr 15, 2015 at 10:26:52, Ian Campbell wrote:
> > > We would like to use memaccess to perform (1) - but rather than
> > Is the guest expected to be aware of this, i.e. to be somewhat
> > paravirtualised? I suppose it must have to be in order to accept
> > seemingly spurious page faults.
> >
> > Which leads me to wonder whether an extra shared ring between the
> > hypervisor and target VCPU would be desirable, i.e. to allow more fine
> > grained semantics than just "computer says no". Specifically if you
> > need to care about the reason for the fault being the actions of an
> > external arbiter rather than some other guest-internal thing.
> >
> > If your application is just to allow the guest OS to kill a process
> > which has tried to touch memory in a way which the external controller
> > has disallowed then a page fault seems like a simple and effective way though.
> 
> The guest will be aware of the permission changes - in fact in our
> system permission changes are only enacted following a request from
> the guest itself.  So, a data abort is sufficient - the guest should
> then be able to work that this was due to it violating its stage-2
> permissions, and kill the appropriate process.

Great, in which case this does seem to be a good approach.
[...]

> It seems to be the simplest approach, and based on some quick prototyping appears to work - at least on ARM.

Excellent!

Ian