Re: [PATCH] KVM: Ensure the exit frequency to QEmu for coalesced MMIO

[Avi Kivity <avi@redhat.com>]

On 01/20/2010 11:34 AM, Sheng Yang wrote:
>
>> I think we can even do this from the I/O thread, without stopping a
>> vcpu, since the colaesced mmio page is not tied to a vcpu but is a vm
>> property.
>>      
> This one sounds better. But I've taken a look at the current userspace code:
>
>    
>> #if defined(KVM_CAP_COALESCED_MMIO)
>>     if (kvm_state->coalesced_mmio) {
>>         struct kvm_coalesced_mmio_ring *ring =
>>             (void *) run + kvm_state->coalesced_mmio * PAGE_SIZE;
>>         while (ring->first != ring->last) {
>>             cpu_physical_memory_rw(ring->coalesced_mmio[ring->first].phys_addr,
>>                            &ring->coalesced_mmio[ring->first].data[0],
>>                            ring->coalesced_mmio[ring->first].len, 1);
>>             smp_wmb();
>>             ring->first = (ring->first + 1) % KVM_COALESCED_MMIO_MAX;
>>         }
>>     }
>> #endif
>>      
> No protection for ring->first and ring->last? Seems it can writing the same element pointed by
> ring->first twice, then skip one element at (ring->first + 1)...
>    

ring->first is owned by userspace, while ring->last is owned by the 
kernel, so no protection is necessary except for the memory barrier.  
Can you elaborate on how it would fail?

-- 
error compiling committee.c: too many arguments to function