linux-kernel - Re: [PATCH V3 2/3] kvm: don't register wildcard MMIO EVENTFD on two buses

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Date:	Mon, 31 Aug 2015 14:33:43 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	gleb@...nel.org, pbonzini@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, cornelia.huck@...ibm.com
Subject: Re: [PATCH V3 2/3] kvm: don't register wildcard MMIO EVENTFD on two
 buses

On Mon, Aug 31, 2015 at 04:03:59PM +0800, Jason Wang wrote:
> 
> 
> On 08/31/2015 03:29 PM, Michael S. Tsirkin wrote:
> >>>>> Thinking more about this, invoking the 0-length write after
> >>>>> > >> > the != 0 length one would be better: it would mean we only
> >>>>> > >> > handle the userspace MMIO like this.
> >>> > > Right.
> >>> > >
> >> > 
> >> > Using current unittest. This patch is about 2.9% slower than before, and
> >> > invoking 0-length write after is still 1.1% slower (mmio-datamatch-eventfd).
> >> > 
> >> > /patch/result/-+%/
> >> > /base/2957/0/
> >> > /V3/3043/+2.9%/
> >> > /V3+invoking != 0 length first/2990/+1.1%/
> >> > 
> >> > So looks like the best method is not searching KVM_FAST_MMIO_BUS during
> >> > KVM_MMIO_BUS. Instead, let userspace to register both datamatch and
> >> > wildcard in this case. Does this sound good to you?
> > No - we can't change userspace.
> 
> Actually, the change was as simple as following. So I don't get the
> reason why.

Because it's too late - we committed to a specific userspace ABI
when this was merged in kernel, we must maintain it.
Even if I thought yours is a good API (and I don't BTW - it's exposing
internal implementation details) it's too late to change it.

> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index 9935029..42ee986 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -288,6 +288,8 @@ static int
> virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
>          if (modern) {
>              memory_region_add_eventfd(modern_mr, modern_addr, 2,
>                                        true, n, notifier);
> +            memory_region_add_eventfd(modern_mr, modern_addr, 0,
> +                                      false, n, notifier);
>          }
>          if (legacy) {
>              memory_region_add_eventfd(legacy_mr, legacy_addr, 2,
> @@ -297,6 +299,8 @@ static int
> virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
>          if (modern) {
>              memory_region_del_eventfd(modern_mr, modern_addr, 2,
>                                        true, n, notifier);
> +            memory_region_del_eventfd(modern_mr, modern_addr, 0,
> +                                      false, n, notifier);
>          }
>          if (legacy) {
>              memory_region_del_eventfd(legacy_mr, legacy_addr, 2,
--
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