Re: KVM: x86: update masterclock when kvmclock_offset is calculated

[Paolo Bonzini <pbonzini@redhat.com>]

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