linux-kernel - Re: [PATCH V5 0/3] introduce coalesced pio support

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Date:   Wed, 17 Oct 2018 11:08:57 +0100
From:   Stefan Hajnoczi <stefanha@...il.com>
To:     Peng Hao <peng.hao2@....com.cn>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH V5 0/3] introduce coalesced pio support

On Sun, Oct 14, 2018 at 07:09:54AM +0800, Peng Hao wrote:
> Coalesced pio is based on coalesced mmio and can be used for some port
> like rtc port, pci-host config port and so on.
> 
> Specially in case of rtc as coalesced pio, 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 pio to handle this scene to reduce VM-EXIT time.
> 
> When starting and closing a virtual machine, it will access pci-host config
> port frequently. So setting these port as coalesced pio can reduce startup 
> and shutdown time. 
> 
> without my patch, get the vm-exit time of accessing rtc 0x70 and piix 0xcf8
> 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%)
> 0xcf8:POUT     1119     2.60%     2.12%   2.79us    56.83us 3.41us (+- 2.23%)
> 
> 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%)
> 0xcf8:POUT      1065    1.67%     0.28%   0.41us    65.44us   0.66us (+- 10.55%)
> 
> 
> Peng Hao (3):
>   kvm/x86 : add coalesced pio support
>   kvm/x86 : add document for coalesced mmio
>   kvm/x86 : add document for coalesced pio
> 
>  Documentation/virtual/kvm/api.txt             | 28 +++++++++++++++++++++++++++
>  include/uapi/linux/kvm.h                      | 11 +++++++++--
>  virt/kvm/coalesced_mmio.c                     | 12 +++++++++---
>  virt/kvm/kvm_main.c                           |  2 ++
>  4 files changed, 48 insertions(+), 5 deletions(-)

Nice, thanks for documenting the API!

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