Re: [Xen-devel] PV guest + shadow mode translate causing hang

[Tim Deegan <tim@xxxxxxx>]

At 08:15 +0000 on 11 Nov (1384154143), Aravindh Puthiyaparambil (aravindp) 
wrote:
> I am trying to implement mem_access for PV guests using shadow pagetables. I 
> am enabling shadow mode for pv using the following call:
> xc_shadow_control(xch, domain_id,  XEN_DOMCTL_SHADOW_OP_ENABLE,  NULL, 0, 
> NULL, 0, NULL);
> 
> This seems to work and I can see a lot of shadow activity if I turn on 
> debugtrace (xl debug T). However I see that the p2m table does not get 
> initialized in this shadow mode. So I tried enabling refcount and translate 
> modes:
> xc_shadow_control(xch, domain_id,  XEN_DOMCTL_SHADOW_OP_ENABLE,  NULL, 0, 
> NULL, XEN_DOMCTL_SHADOW_ENABLE_REFCOUNT | XEN_DOMCTL_SHADOW_ENABLE_TRANSLATE, 
> NULL);
> 
> But this is causing the guest and host to hang.


The guest I expect, as it's not expecting Xen to be translating its
pagetables.  The host hanging is unfortunate. :(

What you want to do is something in between the two: you need to
store some p2m-like information for each PV page but you don't
actually want to do any translation.

One option would be to fix up shadow_mode_translate so that it uses a
1-1 p2m mapping, and then have that mapping be kept up to date with
any alloc/free/map operations.

Another, and I think better, would be to add a third implementation of
the p2m interface, that always return mfn==gfn, but also records
mem_event state.  Then you'd need to add another XEN_DOMCTL_SHADOW
flag to select that mode.

Tim.

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