Re: [RFC PATCH] kvm: x86: reduce rtc 0x70 access vm-exit time

[Paolo Bonzini]

----- Original Message -----
> From: "Peng Hao" <peng.hao2@xxxxxxxxxx>
> To: pbonzini@xxxxxxxxxx, rkrcmar@xxxxxxxxxx
> Cc: kvm@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, "Peng Hao" <peng.hao2@xxxxxxxxxx>
> Sent: Saturday, August 12, 2017 2:06:51 PM
> Subject: [RFC PATCH] kvm: x86: reduce rtc 0x70 access vm-exit time
> 
> some versions of windows guest access rtc frequently because of
> rtc as system tick.guest access rtc like this: write register index
> to 0x70, then write or read data from 0x71. writing 0x70 port is
> just as index and do nothing else. So we can use coalesced mmio to
> handle this scene to reduce VM-EXIT time.
> without my patch, get the vm-exit time of accessing rtc 0x70 using
> perf tools: (guest OS : windows 7 64bit)
> IO Port Access  Samples Samples%  Time%  Min Time  Max Time  Avg time
> 0x70:POUT        86     30.99%    74.59%   9us      29us    10.75us (+-
> 3.41%)
> 
> with my patch
> IO Port Access  Samples Samples%  Time%   Min Time  Max Time   Avg time
>  0x70:POUT       106    32.02%    29.47%    0us      10us     1.57us (+-
>  7.38%)
> 
> the patch is a part of optimizing rtc 0x70 port access. Another is in
> qemu.

Looks good as a proof of concept.  However, you need documentation changes,
and you need to expose this using a capability.  Also, the "pad" field can
be renamed to "pio".

Paolo


> Signed-off-by: Peng Hao <peng.hao2@xxxxxxxxxx>
> ---
>  virt/kvm/coalesced_mmio.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
> index 571c1ce..f640c2f 100644
> --- a/virt/kvm/coalesced_mmio.c
> +++ b/virt/kvm/coalesced_mmio.c
> @@ -82,6 +82,7 @@ static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
>  	ring->coalesced_mmio[ring->last].phys_addr = addr;
>  	ring->coalesced_mmio[ring->last].len = len;
>  	memcpy(ring->coalesced_mmio[ring->last].data, val, len);
> +	ring->coalesced_mmio[ring->last].pad = dev->zone.pad;
>  	smp_wmb();
>  	ring->last = (ring->last + 1) % KVM_COALESCED_MMIO_MAX;
>  	spin_unlock(&dev->kvm->ring_lock);
> @@ -148,8 +149,12 @@ int kvm_vm_ioctl_register_coalesced_mmio(struct kvm
> *kvm,
>  	dev->zone = *zone;
>  
>  	mutex_lock(&kvm->slots_lock);
> -	ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, zone->addr,
> -				      zone->size, &dev->dev);
> +	if (zone->pad == 0)
> +		ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, zone->addr,
> +						zone->size, &dev->dev);
> +	else
> +		ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, zone->addr,
> +						zone->size, &dev->dev);
>  	if (ret < 0)
>  		goto out_free_dev;
>  	list_add_tail(&dev->list, &kvm->coalesced_zones);
> @@ -173,7 +178,10 @@ int kvm_vm_ioctl_unregister_coalesced_mmio(struct kvm
> *kvm,
>  
>  	list_for_each_entry_safe(dev, tmp, &kvm->coalesced_zones, list)
>  		if (coalesced_mmio_in_range(dev, zone->addr, zone->size)) {
> -			kvm_io_bus_unregister_dev(kvm, KVM_MMIO_BUS, &dev->dev);
> +			if (zone->pad == 0)
> +				kvm_io_bus_unregister_dev(kvm, KVM_MMIO_BUS, &dev->dev);
> +			else
> +				kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &dev->dev);
>  			kvm_iodevice_destructor(&dev->dev);
>  		}
>  
> --
> 1.8.3.1
> 
> 
>