Re: A simple question regarding hvm_context_save

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

On 31/05/13 11:46, Jaeyong Yoo wrote:
> Hello,
> I'm reading live migration related code and got a simple question 
> in function hvm_context_save:
>
> static int hvm_save_cpu_ctxt(struct domain *d, hvm_domain_context_t *h)
> {
> blah...
>         /* We don't need to save state for a vcpu that is down; the restore 
>          * code will leave it down if there is nothing saved. */
>         if ( test_bit(_VPF_down, &v->pause_flags) )
>             continue;
> blah...
>
> In the above, if the vcpu is paused, we don't need to store cpu registers
> (since vcpu structure already stored them). But, as far as I know, before
> we save the hvm context, we suspend the domain, which calls shutdown 
> sched_op, and the vcpu is paused. 

_VPF_down is only one possible pause reason.  See
xen/include/xen/sched.h for all of them.

In particular, _VPF_migrating is a different pause reason.

If a VCPU is down, the guest is required to reinitialise it from
scratch.  This is why the state does not need saving.

~Andrew

> Then, I guess testing pause_flags always returns down and we don't
> need to store cpu ctxts. Do I miss something here?
>
> best,
> Jaeyong
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