Re: [RFC] KVM: x86: conditionally acquire/release slots_lock on entry/exit

Avi Kivity
2009-08-27T16:27:23Z

On 08/27/2009 06:54 PM, Marcelo Tosatti wrote:
> perf report shows heavy overhead from down/up of slots_lock.
>
> Attempted to remove slots_lock by having vcpus stop on a synchronization
> point, but this introduced further complexity (a vcpu can be scheduled
> out before reaching the synchronization point, and can sched back in at
> points which are slots_lock protected, etc).
>
> This patch changes vcpu_enter_guest to conditionally release/acquire
> slots_lock in case a vcpu state bit is set.
>
> vmexit performance improves by 5-10% on UP guest.
>

Sorry, it looks pretty complex. Have you considered using srcu? It
seems to me down/up_read() can be replaced by srcu_read_lock/unlock(),
and after proper conversion of memslots and io_bus to
rcu_assign_pointer(), we can just add synchronize_srcu() immediately
after changing stuff (of course mmu_lock still needs to be held when
updating slots).

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.