[Xen-devel] Mem_event API and MEM_EVENT_REASON_SINGLESTEP

[Razvan Cojocaru <rzvncj@xxxxxxxxx>]

Hello,

I'm interested in using the mem_event API (or it's LibVMI wrapper), but 
I'm interested in capturing write events anywhere in a domU guest's 
memory, _and_ not just once for each page write, but every time a page 
is being written to.
To this end, I've looked at the xen-access.c example, where at first all 
the pages are being monitored:
xc_hvm_set_mem_access(xch, domain_id, HVMMEM_access_rx, ~0ull, 0);
xc_hvm_set_mem_access(xch, domain_id, HVMMEM_access_rx, 0, 
xenaccess->domain_info->max_pages);
Then, after an event is received:

xc_hvm_set_mem_access(xch, domain_id, HVMMEM_access_rwx, req.gfn, 1);

thus allowing 'rwx' access to 1 page starting at req.gfn.

This strategy indeed allows me to inspect writes to any page of the 
guest OS, however I can only do this once per page. Once the page is 
allowed HVMMEM_access_rwx access, any subsequent writes to it become 
unavailable to my dom0 userspace tool (I'm "unregistering" for that 
page-specific event).
The author of LibVMI's events code has suggested that I might enable 
writes to that page, single step past the write instruction, and then 
disable writes again.
Two questions:

1. I haven't been able to find an example of how single-stepping via the 
mem_event API might work. Can you point me to some code that does this 
(or a paper, etc.)?
2. Is there any other way to achieve what I'm after?

Thanks,
Razvan Cojocaru

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