Re: How the pv guest responds hypercall_create_continuation ?

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 23/05/13 01:03, Xinxin Jin wrote:
> Hi all, 
>
> 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,
>
>
> Xinxin

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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