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Re: [Xen-devel] [PATCH 1 of 2] Use memops for mem paging, sharing, and access, instead of domctls

> On Wed, 2012-02-01 at 20:09 +0000, Andres Lagar-Cavilla wrote:
>> +int xc_mem_event_memop(xc_interface *xch, domid_t domain_id,
>> +                        unsigned int op, unsigned int mode,
>> +                        uint64_t gfn, void *buffer)
>> +{
>> +    xen_mem_event_op_t meo;
>> +
>> +    memset(&meo, 0, sizeof(meo));
>> +
>> +    meo.op      = op;
>> +    meo.domain  = domain_id;
>> +    meo.gfn     = gfn;
>> +    meo.buffer  = (unsigned long) buffer;
> Hypercall arguments need to use the xc_hypercall_buffer stuff in order
> to ensure that the memory is safe to access from a hypercall (in
> particular that it is mlocked or similar)-- I don't see where that
> happens for this buffer.
> meo itself is bounced by do_memory_op so that is OK.
> I was also a little suspicious of ring_addr and shared_addr in
> xc_mem_event_control in this regard.
> Or does the mem sharing code make it's own arrangements for these sorts
> of things somewhere?

It's rather unclean. They do not get mlock'ed (and there is no sanity
check to ensure they're page-aligned). They should follow the pattern
established in xc_mem_paging_load. I'll get on it, probably a separate

There is a far more insidious problem that I'm not sure how to solve. Say
the pager dies unexpectedly. The page hosting the ring is returned to
dom0's free page pool, and then re-assigned to some other process. Xen
never gets to find out. At best, the domain crashes before an event is
generated. Perhaps, Xen posts at least one event. Worst-case scenario, the
guest generates a spate of events. Cases 2 & 3 corrupt the memory of some
other dom0 process.

I'm glad I got that off my chest :) While being a very rare race condition
(haven't actually experienced it), I am quite unsure of how to tackle it

> Ian.
>> +
>> +    return do_memory_op(xch, mode, &meo, sizeof(meo));
>> +}

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