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kvm/x86 : add document for coalesced mmio 

Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
hifive-unleashed-5.1
Peng Hao 2018-10-14 07:09:56 +08:00 committed by Paolo Bonzini
parent 39337ad1a7
commit 9943450b7b
1 changed files with 24 additions and 0 deletions
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24
Documentation/virtual/kvm/api.txt
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@ -3681,6 +3681,30 @@ Returns: 0 on success, -1 on error
This copies the vcpu's kvm_nested_state struct from userspace to the kernel. For
the definition of struct kvm_nested_state, see KVM_GET_NESTED_STATE.
4.116 KVM_(UN)REGISTER_COALESCED_MMIO
Capability: KVM_CAP_COALESCED_MMIO
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
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
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
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.
5. The kvm_run structure
------------------------