linux-kernel - [PATCH V5 2/3] kvm/x86 : add document for coalesced mmio

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Date:   Sun, 14 Oct 2018 07:09:56 +0800
From:   Peng Hao <peng.hao2@....com.cn>
To:     pbonzini@...hat.com, rkrcmar@...hat.com, stefanha@...il.com
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Peng Hao <peng.hao2@....com.cn>
Subject: [PATCH V5 2/3]  kvm/x86 : add document for coalesced mmio

Signed-off-by: Peng Hao <peng.hao2@....com.cn>
---
 Documentation/virtual/kvm/api.txt | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index 647f941..9615b9e 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -3676,6 +3676,30 @@ 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 shared coalesced
+ring in the kernel.
+
+Coalesced mmio is applied to the following scenario typically.
+If a write access to a hardware register can be deferred, following a
+read/write access to another hardware register on the same device
+will cause the shared coalesced ring to be processed by userspace
+before emulating the current access. That will reduce the first write
+access to userspace.
+
 5. The kvm_run structure
 ------------------------
 
-- 
1.8.3.1