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

[Avi Kivity <avi@redhat.com>]

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.