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From: =?UTF-8?q?Carlos=20L=C3=B3pez?= <clopez@suse.de>
To: linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org,
	linux-doc@vger.kernel.org
Cc: seanjc@google.com,
	=?UTF-8?q?Carlos=20L=C3=B3pez?= <clopez@suse.de>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>
Subject: [PATCH v3] KVM: X86: improve documentation for KVM_CAP_X86_BUS_LOCK_EXIT
Date: Wed, 24 Apr 2024 12:56:18 +0200
Message-Id: <20240424105616.29596-1-clopez@suse.de>
In-Reply-To: <20231116133628.5976-1-clopez@suse.de>
References: <20231116133628.5976-1-clopez@suse.de>
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Improve the description for the KVM_CAP_X86_BUS_LOCK_EXIT capability,
fixing a few typos, grammarm and clarifying the purpose of the ioctl.

Signed-off-by: Carlos López <clopez@suse.de>
---
v3: Added Sean Christopherson's suggestions
v2: Corrected the name of the KVM_RUN_X86_BUS_LOCK flag

 Documentation/virt/kvm/api.rst | 52 ++++++++++++++++++----------------
 1 file changed, 27 insertions(+), 25 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 0b5a33ee71ee..a1d78e06a1ad 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6416,9 +6416,9 @@ More architecture-specific flags detailing state of the VCPU that may
 affect the device's behavior. Current defined flags::
 
   /* x86, set if the VCPU is in system management mode */
-  #define KVM_RUN_X86_SMM     (1 << 0)
+  #define KVM_RUN_X86_SMM          (1 << 0)
   /* x86, set if bus lock detected in VM */
-  #define KVM_RUN_BUS_LOCK    (1 << 1)
+  #define KVM_RUN_X86_BUS_LOCK     (1 << 1)
   /* arm64, set for KVM_EXIT_DEBUG */
   #define KVM_DEBUG_ARCH_HSR_HIGH_VALID  (1 << 0)
 
@@ -7757,29 +7757,31 @@ Valid bits in args[0] are::
   #define KVM_BUS_LOCK_DETECTION_OFF      (1 << 0)
   #define KVM_BUS_LOCK_DETECTION_EXIT     (1 << 1)
 
-Enabling this capability on a VM provides userspace with a way to select
-a policy to handle the bus locks detected in guest. Userspace can obtain
-the supported modes from the result of KVM_CHECK_EXTENSION and define it
-through the KVM_ENABLE_CAP.
-
-KVM_BUS_LOCK_DETECTION_OFF and KVM_BUS_LOCK_DETECTION_EXIT are supported
-currently and mutually exclusive with each other. More bits can be added in
-the future.
-
-With KVM_BUS_LOCK_DETECTION_OFF set, bus locks in guest will not cause vm exits
-so that no additional actions are needed. This is the default mode.
-
-With KVM_BUS_LOCK_DETECTION_EXIT set, vm exits happen when bus lock detected
-in VM. KVM just exits to userspace when handling them. Userspace can enforce
-its own throttling or other policy based mitigations.
-
-This capability is aimed to address the thread that VM can exploit bus locks to
-degree the performance of the whole system. Once the userspace enable this
-capability and select the KVM_BUS_LOCK_DETECTION_EXIT mode, KVM will set the
-KVM_RUN_BUS_LOCK flag in vcpu-run->flags field and exit to userspace. Concerning
-the bus lock vm exit can be preempted by a higher priority VM exit, the exit
-notifications to userspace can be KVM_EXIT_BUS_LOCK or other reasons.
-KVM_RUN_BUS_LOCK flag is used to distinguish between them.
+Enabling this capability on a VM provides userspace with a way to select a
+policy to handle the bus locks detected in guest. Userspace can obtain the
+supported modes from the result of KVM_CHECK_EXTENSION and define it through
+the KVM_ENABLE_CAP. The supported modes are mutually-exclusive.
+
+This capability allows userspace to force VM exits on bus locks detected in the
+guest, irrespective whether or not the host has enabled split-lock detection
+(which triggers an #AC exception that KVM intercepts). This capability is
+intended to mitigate attacks where a malicious/buggy guest can exploit bus
+locks to degrade the performance of the whole system.
+
+If KVM_BUS_LOCK_DETECTION_OFF is set, KVM doesn't force guest bus locks to VM
+exit, although the host kernel's split-lock #AC detection still applies, if
+enabled.
+
+If KVM_BUS_LOCK_DETECTION_EXIT is set, KVM enables a CPU feature that ensures
+bus locks in the guest trigger a VM exit, and KVM exits to userspace for all
+such VM exits, e.g. to allow userspace to throttle the offending guest and/or
+apply some other policy-based mitigation. When exiting to userspace, KVM sets
+KVM_RUN_X86_BUS_LOCK in vcpu-run->flags, and conditionally sets the exit_reason
+to KVM_EXIT_X86_BUS_LOCK.
+
+Note! Detected bus locks may be coincident with other exits to userspace, i.e.
+KVM_RUN_X86_BUS_LOCK should be checked regardless of the primary exit reason if
+userspace wants to take action on all detected bus locks.
 
 7.23 KVM_CAP_PPC_DAWR1
 ----------------------
-- 
2.35.3