Xen devel - Dec 2012 - Handling a MEM_EVENT_REASON_CR3

Hello,

a few questions about receiving a MEM_EVENT_REASON_CR3 even in dom0 
userspace:

1. If I call

xc_set_hvm_param(xci, domain_id,
                  HVM_PARAM_MEMORY_EVENT_CR3,
                  HVMPME_onchangeonly);

that only triggers a CR3 event if the new value that the guest writes to 
CR3 is different from the existing value, is that assumption correct?

2. mem_event.h says that if "CR3 was hit: gfn is CR3 value".
I''m
assuming that gfn is the _new_ value, and that the old value is 
unavailable, is that also correct?

3. Is it possible to, upon intercepting the CR3 write, write a 
_different_ value to CR3, instead of gfn?

Thanks,
Razvan Cojocaru

At 16:16 +0200 on 12 Dec (1355329007), Razvan Cojocaru
wrote:
> Hello,
> 
> a few questions about receiving a MEM_EVENT_REASON_CR3 even in dom0 
> userspace:
> 
> 1. If I call
> 
> xc_set_hvm_param(xci, domain_id,
>                  HVM_PARAM_MEMORY_EVENT_CR3,
>                  HVMPME_onchangeonly);
> 
> that only triggers a CR3 event if the new value that the guest writes to 
> CR3 is different from the existing value, is that assumption correct?
Yes, I think so.  That''s what HVMPME_onchangeonly ought to do.
Did you try it?
> 2. mem_event.h says that if "CR3 was hit: gfn is CR3 value".
I''m
> assuming that gfn is the _new_ value, and that the old value is 
> unavailable, is that also correct?
Yes.
> 3. Is it possible to, upon intercepting the CR3 write, write a 
> _different_ value to CR3, instead of gfn?
Yes.  you can use either xc_vcpu_getcontext()/xc_vcpu_setcontext()
or xc_domain_hvm_getcontext_partial()/xc_domain_hvm_setcontext() to read
and write the vCPU registers.

Cheers,

Tim.