Re: [PATCH] kvm tools: Add MMIO coalescing support

[Ingo Molnar <mingo@elte.hu>]

* Alexander Graf <agraf@suse.de> wrote:

> 
> On 04.06.2011, at 16:46, Ingo Molnar wrote:
> 
> > 
> > * Alexander Graf <agraf@suse.de> wrote:
> > 
> >> So the simple rule is: don't register a coalesced MMIO region for a 
> >> region where latency matters. [...]
> > 
> > So my first suspicion is confirmed.
> > 
> > A quick look at Qemu sources shows that lots of drivers are using 
> > coalesced_mmio without being aware of the latency effects and only 
> > one seems to make use of qemu_flush_coalesced_mmio_buffer(). Drivers 
> > like hw/e1000.c sure look latency critical to me.
> 
> e1000 maps its NVRAM on coalesced mmio - which is completely ok.

Ok!

> > So i maintain my initial opinion: this is a pretty dangerous 
> > 'optimization' that should be used with extreme care: i can tell 
> > it you with pretty good authority that latency problems are much 
> > more easy to introduce than to find and remove ...
> 
> Yup, which is why it's very sparsely used in qemu :). Basically, 
> it's only e1000 and vga, both of which are heavily used and tested 
> drivers.

Ok, so this change in:

 commit 73389b5ea017288a949ae27536c8cfd298d3e317
 Author: Sasha Levin <levinsasha928@gmail.com>
 Date:   Fri Jun 3 22:51:08 2011 +0300

    kvm tools: Add MMIO coalescing support

@@ -67,6 +70,16 @@ bool kvm__register_mmio(u64 phys_addr, u64 phys_addr_len, void (*kvm_mmio_callba
                .kvm_mmio_callback_fn = kvm_mmio_callback_fn,
        };
 
+       zone = (struct kvm_coalesced_mmio_zone) {
+               .addr   = phys_addr,
+               .size   = phys_addr_len,
+       };
+       ret = ioctl(kvm->vm_fd, KVM_REGISTER_COALESCED_MMIO, &zone);
+       if (ret < 0) {
+               free(mmio);
+               return false;
+       }

Seems completely wrong, because it indiscriminately registers *all* 
mmio regions as coalesced ones.

Thanks,

	Ingo