Re: [RFC PATCH v3 01/11] KVM: Capture VM start

On Fri, Jan 7, 2022 at 5:06 PM Sean Christopherson [off-list ref] wrote:

On Tue, Jan 04, 2022, Raghavendra Rao Ananta wrote:

quoted

Capture the start of the KVM VM, which is basically the

Please wrap at ~75 chars.

quoted

start of any vCPU run. This state of the VM is helpful
in the upcoming patches to prevent user-space from
configuring certain VM features after the VM has started
running.

Please provide context of how the flag will be used.  I glanced at the future
patches, and knowing very little about arm, I was unable to glean useful info
about exactly who is being prevented from doing what.

quoted

Signed-off-by: Raghavendra Rao Ananta <redacted>
---
 include/linux/kvm_host.h | 3 +++
 virt/kvm/kvm_main.c      | 9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index c310648cc8f1..d0bd8f7a026c 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h

@@ -623,6 +623,7 @@ struct kvm {
      struct notifier_block pm_notifier;
 #endif
      char stats_id[KVM_STATS_NAME_SIZE];
+     bool vm_started;
 };

 #define kvm_err(fmt, ...) \

@@ -1666,6 +1667,8 @@ static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
      }
 }

+#define kvm_vm_has_started(kvm) (kvm->vm_started)

Needs parantheses around (kvm), but why bother with a macro?  This is the same
header that defines struct kvm.

No specific reason for creating a macro as such. I can remove it if it
feels noisy.

quoted

+
 extern bool kvm_rebooting;

 extern unsigned int halt_poll_ns;

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 72c4e6b39389..962b91ac2064 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c

@@ -3686,6 +3686,7 @@ static long kvm_vcpu_ioctl(struct file *filp,
      int r;
      struct kvm_fpu *fpu = NULL;
      struct kvm_sregs *kvm_sregs = NULL;
+     struct kvm *kvm = vcpu->kvm;

If you're going to bother grabbing kvm, replace the instances below that also do
vcpu->kvm.

quoted

      if (vcpu->kvm->mm != current->mm || vcpu->kvm->vm_dead)
              return -EIO;

@@ -3723,6 +3724,14 @@ static long kvm_vcpu_ioctl(struct file *filp,
                      if (oldpid)
                              synchronize_rcu();
                      put_pid(oldpid);
+
+                     /*
+                      * Since we land here even on the first vCPU run,
+                      * we can mark that the VM has started running.

Please avoid "we", "us", etc..

"vm_started" is also ambiguous.  If we end up with a flag, then I would prefer a
much more literal name, a la created_vcpus, e.g. ran_vcpus or something.

quoted

+                      */
+                     mutex_lock(&kvm->lock);

This adds unnecessary lock contention when running vCPUs.  The naive solution
would be:
                        if (!kvm->vm_started) {
                                ...
                        }

Not sure if I understood the solution..

quoted

+                     kvm->vm_started = true;
+                     mutex_unlock(&kvm->lock);

Lastly, why is this in generic KVM?

The v1 of the series originally had it in the arm specific code.
However, I was suggested to move it to the generic code since the book
keeping is not arch specific and could be helpful to others too [1].

Thanks for the review. I'll add your other comments as well.

Regards,
Raghavendra

[1]: https://lore.kernel.org/kvmarm/YYMKphExkqttn2w0@google.com/ (local)

quoted

              }
              r = kvm_arch_vcpu_ioctl_run(vcpu);
              trace_kvm_userspace_exit(vcpu->run->exit_reason, r);
--
2.34.1.448.ga2b2bfdf31-goog

_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel