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Re: [Xen-devel] [PATCH V6] x86/altp2m: Hypercall to set altp2m view visibility

> From: Isaila Alexandru <aisaila@xxxxxxxxxxxxxxx>
> Sent: Tuesday, March 24, 2020 6:46 PM
> Hi Kevin and sorry for the long reply time,
> On 10.03.2020 04:04,  sTian, Kevin wrote:
> >> From: Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>
> >> Sent: Tuesday, March 3, 2020 8:23 PM
> >>
> >> At this moment a guest can call vmfunc to change the altp2m view. This
> >> should be limited in order to avoid any unwanted view switch.
> >
> > I look forward to more elaboration of the motivation, especially for one
> > who doesn't track altp2m closely like me. For example, do_altp2m_op
> > mentions three modes: external, internal, coordinated. Then is this patch
> > trying to limit the view switch in all three modes or just one of them?
> > from the definition clearly external disallows guest to change any view
> > (then why do we want per-view visibility control) while the latter two
> > both allows guest to switch the view. later you noted some exception
> > with mixed (internal) mode. then is this restriction pushed just for
> > limited (coordinated) mode?
> >
> As you stated, there are some exceptions with mixed (internal) mode.
> This restriction is clearly used for coordinated mode but it also
> restricts view switching in the external mode as well. I had a good
> example to start with, let's say we have one external agent in dom0 that
> uses view1 and view2 and the logic requires the switch between the
> views. At this point VMFUNC is available to the guest so with a simple
> asm code it can witch to view 0. At this time the external agent is not
> aware that the view has switched and further more view0 was not supposed
> to be in the main logic so it crashes. This example can be extended to
> any number of views. I hope it can paint a more clear picture of what
> this patch is trying to achive.

Can VMFUNC be hidden and disabled when external mode is being used?
or is it because the mode can be dynamically switched after a VM is 
launched so you have to restrict the views in this way?

> > btw I'm not sure why altp2m invents two names per mode, and their
> > mapping looks a bit weird. e.g. isn't 'coordinated' mode sound more
> > like 'mixed' mode?
> Yes that is true, it si a bit weird.
> >
> >>
> >> The new xc_altp2m_set_visibility() solves this by making views invisible
> >> to vmfunc.
> >
> > if one doesn't want to make view visible to vmfunc, why can't he just
> > avoids registering the view at the first place? Are you aiming for a
> > scenario that dom0 may register 10 views, with 5 views visible to
> > vmfunc with the other 5 views switched by dom0 itself?
> That is one scenario, another can be that dom0 has a number of views
> created and in some time it wants to be sure that only some of the views
> can be switched, saving the rest and making them visible when the time
> is right. Sure the example given up is another example.

Can you update the patch description and resend? I'll take another look then.




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