Il 03/09/2013 05:03, Marcelo Tosatti ha scritto:
> > Here is the code I was thinking of:
> >
> > spin_lock(&ka->pvclock_gtod_sync_lock);
> > make_all_cpus_request(kvm, KVM_REQ_CLOCK_UPDATE);
> >
> > /*
> > * No guest entries from this point: VCPUs will be spinning
> > * on pvclock_gtod_sync_lock in kvm_guest_time_update.
> > */
> > pvclock_update_vm_gtod_copy(kvm);
> >
> > /*
> > * Let kvm_guest_time_update continue: entering the guest
> > * is now allowed too.
> > */
> > spin_unlock(&ka->pvclock_gtod_sync_lock);
> >
> > KVM_REQ_CLOCK_UPDATE is used to cancel guest entry and execute
> > kvm_guest_time_update. But kvm_guest_time_update will spin on
> > pvclock_gtod_sync_lock until pvclock_update_vm_gtod_copy exits and
> > kvm_gen_update_masterclock releases the spinlock.
>
> Not safe because there are places which set KVM_REQ_CLOCK_UPDATE without
> kicking target vcpu out of guest mode. Unless you use a modified
> make_all_cpus_request.
make_all_cpus_request does force an exit, even if the bit is already set
in vcpus->requests. But maybe here is where I'm missing something.
> The point of REQ_MCLOCK_INPROGRESS request is to guarantee that the
> following is not possible:
>
> - 2 vcpus in guest mode with per-vcpu kvmclock areas with
> different {system_timestamp, tsc_offset} values.
Understood.
> To achieve that:
>
> - Kick all vcpus out of guest mode (via a request bit that can't be
> cleared).
> - Update the {system_timestamp, tsc_offset} values.
> - Clear the request bit.
After make_all_cpus_request, all VCPUs will be out of guest mode, and
the request bit will not be cleared until pvclock_gtod_sync_lock is
released.
Paolo
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