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

["Aravindh Puthiyaparambil (aravindp)" <aravindp@xxxxxxxxx>]

>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.

OK, I will go down the path of adding another shadow mode. 

There is one other thing I am not clear about. When the mem_access listener is 
trying to map guest memory or set access permissions for say a 256MB pv guest, 
does it make the calls on gfns ranging from 0 to 256mb or does it need to do it 
on the mfn? Or should this gnf -> mfn translation be abstracted by the new 
shadow mode for the mem_access listener?

Thanks,
Aravindh


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