Re: [PATCH v2] KVM: VMX: Remove redundant handling of bus lock vmexit

From: Xiaoyao Li
Date: Fri Oct 15 2021 - 04:43:46 EST


On 10/15/2021 3:28 PM, Hao Xiang wrote:


On 2021/10/15 14:46, Xiaoyao Li wrote:
On 10/15/2021 11:57 AM, Hao Xiang wrote:
Hardware may or may not set exit_reason.bus_lock_detected on BUS_LOCK
VM-Exits. Dealing with KVM_RUN_X86_BUS_LOCK in handle_bus_lock_vmexit
could be redundant when exit_reason.basic is EXIT_REASON_BUS_LOCK.

We can remove redundant handling of bus lock vmexit. Force Setting
exit_reason.bus_lock_detected in handle_bus_lock_vmexit(), and deal with
KVM_RUN_X86_BUS_LOCK only in vmx_handle_exit().

Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Hao Xiang <hao.xiang@xxxxxxxxxxxxxxxxx>
---
v1 -> v2: a little modifications of comments

  arch/x86/kvm/vmx/vmx.c | 15 +++++++++------
  1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 116b089..22be02e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5562,9 +5562,13 @@ static int handle_encls(struct kvm_vcpu *vcpu)
  static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
  {
-    vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
-    vcpu->run->flags |= KVM_RUN_X86_BUS_LOCK;
-    return 0;
+    /*
+     * Hardware may or may not set the BUS_LOCK_DETECTED flag on BUS_LOCK
+     * VM-Exits, force setting the flag

         so that the logic in vmx_handle_exit()
+     * doesn't have to handle the flag and the basic exit reason.

The code itself looks good.

But the comment is partly incorrect. Now we rely only on the bus_lock_detected flag so vmx_handle_exit() must check the flag.
Thank you for you advice. Is the following comment ok?
/*
 * Hardware may or may not set the BUS_LOCK_DETECTED flag on BUS_LOCK
 * VM-Exits. Forcely set the flag here, then check the flag in
 * vmx_handle_exit() and handle the bus lock vmexit if the flag is
 * set.
 */

(I'm not sure if forcely is a valid word)

how about

/*
* Hardware may or may not set the BUS_LOCK_DETECTED flag on BUS_LOCK
* VM Exit. Unconditionally set the flag here and leave the handling to
* vmx_handle_exit().
*/

+     */
+    to_vmx(vcpu)->exit_reason.bus_lock_detected = true;
+    return 1;
  }
  /*
@@ -6051,9 +6055,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
      int ret = __vmx_handle_exit(vcpu, exit_fastpath);
      /*
-     * Even when current exit reason is handled by KVM internally, we
-     * still need to exit to user space when bus lock detected to inform
-     * that there is a bus lock in guest.
+     * Exit to user space when bus lock detected to inform that there is
+     * a bus lock in guest.
       */
      if (to_vmx(vcpu)->exit_reason.bus_lock_detected) {
          if (ret > 0)