• Peng Hao's avatar
    kvm/x86 : add coalesced pio support · 0804c849
    Peng Hao authored
    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%)
    Signed-off-by: default avatarPeng Hao <peng.hao2@zte.com.cn>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    0804c849
api.txt 160 KB