Message-ID: <20220831001706.4075399-4-seanjc@google.com>
Date:   Wed, 31 Aug 2022 00:17:06 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Aaron Lewis <aaronlewis@...gle.com>
Subject: [PATCH 3/3] KVM: x86: Clean up KVM_CAP_X86_USER_SPACE_MSR documentation

Clean up the KVM_CAP_X86_USER_SPACE_MSR documentation to eliminate
misleading and/or inconsistent verbiage, and to actually document what
accesses are intercepted by which flags.

  - s/will/may since not all #GPs are guaranteed to be intercepted
  - s/deflect/intercept to align with common KVM terminology
  - s/user space/userspace to align with the majority of KVM docs
  - Avoid using "trap" terminology, as KVM exits to userspace _before_
    stepping, i.e. doesn't exhibit trap-like behavior
  - Actually document the flags

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 Documentation/virt/kvm/api.rst | 40 ++++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 16 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 406d3e7c5a59..32d8fc8dcbb5 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6431,29 +6431,29 @@ if it decides to decode and emulate the instruction.
 
 Used on x86 systems. When the VM capability KVM_CAP_X86_USER_SPACE_MSR is
 enabled, MSR accesses to registers that would invoke a #GP by KVM kernel code
-will instead trigger a KVM_EXIT_X86_RDMSR exit for reads and KVM_EXIT_X86_WRMSR
+may instead trigger a KVM_EXIT_X86_RDMSR exit for reads and KVM_EXIT_X86_WRMSR
 exit for writes.
 
-The "reason" field specifies why the MSR trap occurred. User space will only
-receive MSR exit traps when a particular reason was requested during through
+The "reason" field specifies why the MSR interception occurred. Userspace will
+only receive MSR exits when a particular reason was requested during through
 ENABLE_CAP. Currently valid exit reasons are:
 
 	KVM_MSR_EXIT_REASON_UNKNOWN - access to MSR that is unknown to KVM
 	KVM_MSR_EXIT_REASON_INVAL - access to invalid MSRs or reserved bits
 	KVM_MSR_EXIT_REASON_FILTER - access blocked by KVM_X86_SET_MSR_FILTER
 
-For KVM_EXIT_X86_RDMSR, the "index" field tells user space which MSR the guest
-wants to read. To respond to this request with a successful read, user space
+For KVM_EXIT_X86_RDMSR, the "index" field tells userspace which MSR the guest
+wants to read. To respond to this request with a successful read, userspace
 writes the respective data into the "data" field and must continue guest
 execution to ensure the read data is transferred into guest register state.
 
-If the RDMSR request was unsuccessful, user space indicates that with a "1" in
+If the RDMSR request was unsuccessful, userspace indicates that with a "1" in
 the "error" field. This will inject a #GP into the guest when the VCPU is
 executed again.
 
-For KVM_EXIT_X86_WRMSR, the "index" field tells user space which MSR the guest
-wants to write. Once finished processing the event, user space must continue
-vCPU execution. If the MSR write was unsuccessful, user space also sets the
+For KVM_EXIT_X86_WRMSR, the "index" field tells userspace which MSR the guest
+wants to write. Once finished processing the event, userspace must continue
+vCPU execution. If the MSR write was unsuccessful, userspace also sets the
 "error" field to "1".
 
 See KVM_X86_SET_MSR_FILTER for details on the interaction with MSR filtering.
@@ -7223,19 +7223,27 @@ the module parameter for the target VM.
 :Parameters: args[0] contains the mask of KVM_MSR_EXIT_REASON_* events to report
 :Returns: 0 on success; -1 on error
 
-This capability enables trapping of #GP invoking RDMSR and WRMSR instructions
-into user space.
+This capability allows userspace to intercept RDMSR and WRMSR instructions if
+access to an MSR is denied.  By default, KVM injects #GP on denied accesses.
 
 When a guest requests to read or write an MSR, KVM may not implement all MSRs
 that are relevant to a respective system. It also does not differentiate by
 CPU type.
 
-To allow more fine grained control over MSR handling, user space may enable
+To allow more fine grained control over MSR handling, userspace may enable
 this capability. With it enabled, MSR accesses that match the mask specified in
-args[0] and trigger a #GP event inside the guest by KVM will instead trigger
-KVM_EXIT_X86_RDMSR and KVM_EXIT_X86_WRMSR exit notifications which user space
-can then handle to implement model specific MSR handling and/or user notifications
-to inform a user that an MSR was not handled.
+args[0] and would trigger a #GP inside the guest will instead trigger
+KVM_EXIT_X86_RDMSR and KVM_EXIT_X86_WRMSR exit notifications.  Userspace
+can then implement model specific MSR handling and/or user notifications
+to inform a user that an MSR was not emulated/virtualized by KVM.
+
+The valid mask flags are:
+
+	KVM_MSR_EXIT_REASON_UNKNOWN - intercept accesses to unknown (to KVM) MSRs
+	KVM_MSR_EXIT_REASON_INVAL   - intercept accesses that are architecturally
+                                invalid according to the vCPU model and/or mode
+	KVM_MSR_EXIT_REASON_FILTER  - intercept accesses that are denied by userspace
+                                via KVM_X86_SET_MSR_FILTER
 
 7.22 KVM_CAP_X86_BUS_LOCK_EXIT
 -------------------------------
-- 
2.37.2.672.g94769d06f0-goog

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