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On 23/05/13 01:03, Xinxin Jin wrote:
Hi all,Â
In Xen's hypercall
handler, sometimes when -EAGAIN error happens, Xen will call a
function hypercall_create_continuation(), which writes the
arguments of this hypercall to guest user registers. I guess
the purpose of this function is to indicate guest to try the
failed hypercall again, right ? Then I looked at guest Linux's
code and try to find the interface:
A typical guest
hypercall calling routine is (take HYPERVISOR_mmu_update for
example):
static inline int
HYPERVISOR_mmu_update(
  mmu_update_t *req, int count, int *success_count,
domid_t domid)
{
  return _hypercall4(int, mmu_update, req, count,
success_count, domid);
}
Then _hypercall4 will call corresponding code in
hypercall_page and return. But where is the interface with
hypercall_create_continuation()? How the guest knows it
needs to try the hypercall again?
Thanks a lot,
The whole point of hypercall continuations is to reduce the time
spent in the hypervisor without processing softirqs/events etc.
Therefore, long running hypercall operations create continuations,
allowing a small chunk of work to be done now, and the next chunk of
work to be done when the VCPU is next scheduled, which might be
right afterwards but might be in a long times time.
The mechanism for creating a continuation is to store the
intermediate state in the guest registers and subtract 2 from the
guests EIP, causing it to re-execute the int 0x82/sysenter/syscall
instruction when next scheduled.
~Andrew
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