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@xxxxxxx> --- 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
