Re: [PATCH] IO: Intelligent device lookup on bus

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-07-20 14:11, Sasha Levin wrote:
> Currently the method of dealing with an IO operation on a bus (PIO/MMIO)
> is to call the read or write callback for each device registered
> on the bus until we find a device which handles it.
> 
> Since the number of devices on a bus can be significant due to ioeventfds
> and coalesced MMIO zones, this leads to a lot of overhead on each IO
> operation.
> 
> Instead of registering devices, we now register ranges which points to
> a device. Lookup is done using an efficient bsearch instead of a linear
> search.
> 
> This should speed up all IO operations generated by the guest.
> 
> Cc: Avi Kivity <avi@redhat.com>
> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
> ---
> 
> This patch depends on '[PATCH v3] MMIO: Make coalesced mmio use a device
> per zone'.
> 
>  arch/x86/kvm/i8254.c      |    4 +-
>  arch/x86/kvm/i8259.c      |    4 +-
>  include/linux/kvm_host.h  |   18 ++++----
>  virt/kvm/coalesced_mmio.c |    6 +--
>  virt/kvm/eventfd.c        |    3 +-
>  virt/kvm/ioapic.c         |   13 +-----
>  virt/kvm/kvm_main.c       |  107 +++++++++++++++++++++++++++++++++++++++-----
>  7 files changed, 115 insertions(+), 40 deletions(-)
> 
> diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
> index efad723..61d193c 100644
> --- a/arch/x86/kvm/i8254.c
> +++ b/arch/x86/kvm/i8254.c
> @@ -713,13 +713,15 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
>  	kvm_register_irq_mask_notifier(kvm, 0, &pit->mask_notifier);
>  
>  	kvm_iodevice_init(&pit->dev, &pit_dev_ops);
> -	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, &pit->dev);
> +	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, KVM_PIT_BASE_ADDRESS,
> +					KVM_PIT_MEM_LENGTH, &pit->dev);
>  	if (ret < 0)
>  		goto fail;
>  
>  	if (flags & KVM_PIT_SPEAKER_DUMMY) {
>  		kvm_iodevice_init(&pit->speaker_dev, &speaker_dev_ops);
>  		ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS,
> +						KVM_SPEAKER_BASE_ADDRESS, 4,
>  						&pit->speaker_dev);
>  		if (ret < 0)
>  			goto fail_unregister;
> diff --git a/arch/x86/kvm/i8259.c b/arch/x86/kvm/i8259.c
> index 19fe855..c2295af 100644
> --- a/arch/x86/kvm/i8259.c
> +++ b/arch/x86/kvm/i8259.c
> @@ -562,7 +562,9 @@ struct kvm_pic *kvm_create_pic(struct kvm *kvm)
>  	 */
>  	kvm_iodevice_init(&s->dev, &picdev_ops);
>  	mutex_lock(&kvm->slots_lock);
> -	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, &s->dev);
> +	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, 0x20, 2, &s->dev);
> +	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, 0xa0, 2, &s->dev);
> +	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, 0x4d0, 2, &s->dev);

This made me wonder if you are incorrectly registering the same device
multiple times. kvm_io_bus_register_dev is no longer optimally
describing what is done here: the registration of an IO range with the
bus. And that range is handled by a specific device. Conceptually, a
device is only attached once to some bus.

Jan

-- 
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