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authorPeng Hao <peng.hao2@zte.com.cn>2018-10-14 07:09:55 +0800
committerPaolo Bonzini <pbonzini@redhat.com>2018-10-17 00:30:11 +0200
commit0804c849f1df0992d39a37c4fc259f7f8b16f385 (patch)
tree40ab26eea50c1c904522352d56870beb6126b20b /Documentation/virtual
parent9943450b7b8831c5045362eed45f6fefd1986d72 (diff)
kvm/x86 : add coalesced pio support
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: Peng Hao <peng.hao2@zte.com.cn> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'Documentation/virtual')
-rw-r--r--Documentation/virtual/kvm/api.txt16
1 files changed, 10 insertions, 6 deletions
diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index 10d48eb67da9..70f9c8bb1840 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -3683,27 +3683,31 @@ the definition of struct kvm_nested_state, see KVM_GET_NESTED_STATE.
4.116 KVM_(UN)REGISTER_COALESCED_MMIO
-Capability: KVM_CAP_COALESCED_MMIO
+Capability: KVM_CAP_COALESCED_MMIO (for coalesced mmio)
+ KVM_CAP_COALESCED_PIO (for coalesced pio)
Architectures: all
Type: vm ioctl
Parameters: struct kvm_coalesced_mmio_zone
Returns: 0 on success, < 0 on error
-Coalesced mmio is a performance optimization that defers hardware
+Coalesced I/O is a performance optimization that defers hardware
register write emulation so that userspace exits are avoided. It is
typically used to reduce the overhead of emulating frequently accessed
hardware registers.
-When a hardware register is configured for coalesced mmio, write accesses
+When a hardware register is configured for coalesced I/O, write accesses
do not exit to userspace and their value is recorded in a ring buffer
that is shared between kernel and userspace.
-Coalesced mmio is used if one or more write accesses to a hardware
+Coalesced I/O is used if one or more write accesses to a hardware
register can be deferred until a read or a write to another hardware
register on the same device. This last access will cause a vmexit and
userspace will process accesses from the ring buffer before emulating
-it. That will avoid exiting to userspace on repeated writes to the
-first register.
+it. That will avoid exiting to userspace on repeated writes.
+
+Coalesced pio is based on coalesced mmio. There is little difference
+between coalesced mmio and pio except that coalesced pio records accesses
+to I/O ports.
5. The kvm_run structure
------------------------