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