[Xen-devel] Details on p2m_mem_access_resume()

[Razvan Cojocaru <rzvncj@xxxxxxxxx>]

Hello,

I've seen that, once the hypervisor code decides that it should send a 
mem_event to userspace in p2m_mem_access_check() (file: 
xen/arch/x86/mm/p2m.c), it first pauses the VCPU that tried to write to 
a page that doesn't allow writes. Then, control is handed over to 
userspace, and then back to the hypervisor via p2m_mem_access_resume(), 
which looks like this:
void p2m_mem_access_resume(struct domain *d)
{
    mem_event_response_t rsp;

    /* Pull all responses off the ring */
    while( mem_event_get_response(d, &d->mem_event->access, &rsp) )
    {
        if ( rsp.flags & MEM_EVENT_FLAG_DUMMY )
            continue;
        /* Unpause domain */
        if ( rsp.flags & MEM_EVENT_FLAG_VCPU_PAUSED )
            vcpu_unpause(d->vcpu[rsp.vcpu_id]);
    }
}

What I want to do is: based on the response received from userspace 
(i.e. if ( rsp.flags & MY_FLAG ) ...), allow the write instruction 
(without unprotecting the page) and move on. This means: I want to be 
able to receive a page fault mem_event caused by a write only once, 
_without_ lifting the write restrictions.
I've tried to achieve this by using hvm_emulate_one(), but in order to 
be able to use it, I need a valid struct cpu_user_regs *parameter. As 
I've written to xen-devel in a previous thread, I've initially tried to 
obtain that via guest_cpu_user_regs(), which was not the way to go. I've 
recently tried &(d->vcpu[rsp.vcpu_id]->arch.user_regs) instead, with 
similar success. I've even passed a valid struct cpu_user_regs 
*parameter to p2m_mem_access_check(), from ept_handle_violation() down, 
and kept that pointer to use later on from p2m_mem_access_resume(). 
Nothing has worked so far.
Is it possible, in theory, to achieve what I'm after? Is there another 
way I might be able to achieve it, if emulating the write instruction is 
not possible there?
How does p2m_mem_access_resume() fit into the overall code flow, and 
what can (and can't) I do from it's scope, related to the specific vcpu 
I'm interested in?
Thank you in advance for your replies,
Razvan Cojocaru

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