[PATCH 4/4] KVM: use signals to abort enter_guest/blocking and retry

Emanuele Giuseppe Esposito
2022-10-22T15:48:31Z

Once a vcpu exectues KVM_RUN, it could enter two states:
enter guest mode, or block/halt.
Use a signal to allow a vcpu to exit the guest state or unblock,
so that it can exit KVM_RUN and release the read semaphore,
allowing a pending KVM_KICK_ALL_RUNNING_VCPUS to continue.

Note that the signal is not deleted and used to propagate the
exit reason till vcpu_run(). It will be clearead only by
KVM_RESUME_ALL_KICKED_VCPUS. This allows the vcpu to keep try
entering KVM_RUN and perform again all checks done in
kvm_arch_vcpu_ioctl_run() before entering the guest state,
where it will return again if the request is still set.

However, the userspace hypervisor should also try to avoid
continuously calling KVM_RUN after invoking KVM_KICK_ALL_RUNNING_VCPUS,
because such call will just translate in a back-to-back down_read()
and up_read() (thanks to the signal).

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
arch/x86/include/asm/kvm_host.h | 2 ++
arch/x86/kvm/x86.c | 8 ++++++++
virt/kvm/kvm_main.c | 21 +++++++++++++++++++++
3 files changed, 31 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index aa381ab69a19..d5c37f344d65 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -108,6 +108,8 @@
KVM_ARCH_REQ_FLAGS(30, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
#define KVM_REQ_MMU_FREE_OBSOLETE_ROOTS \
KVM_ARCH_REQ_FLAGS(31, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
+#define KVM_REQ_USERSPACE_KICK \
+ KVM_ARCH_REQ_FLAGS(32, KVM_REQUEST_WAIT)

#define CR0_RESERVED_BITS \
(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b0c47b41c264..2af5f427b4e9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10270,6 +10270,10 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
}

if (kvm_request_pending(vcpu)) {
+ if (kvm_test_request(KVM_REQ_USERSPACE_KICK, vcpu)) {
+ r = -EINTR;
+ goto out;
+ }
if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu)) {
r = -EIO;
goto out;
@@ -10701,6 +10705,10 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
r = vcpu_block(vcpu);
}

+ /* vcpu exited guest/unblocked because of this request */
+ if (kvm_test_request(KVM_REQ_USERSPACE_KICK, vcpu))
+ return -EINTR;
+
if (r <= 0)
break;

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ae0240928a4a..13fa7229b85d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3431,6 +3431,8 @@ static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
goto out;
if (kvm_check_request(KVM_REQ_UNBLOCK, vcpu))
goto out;
+ if (kvm_test_request(KVM_REQ_USERSPACE_KICK, vcpu))
+ goto out;

ret = 0;
out:
@@ -4668,6 +4670,25 @@ static long kvm_vm_ioctl(struct file *filp,
r = kvm_vm_ioctl_enable_cap_generic(kvm, &cap);
break;
}
+ case KVM_KICK_ALL_RUNNING_VCPUS: {
+ /*
+ * Notify all running vcpus that they have to stop.
+ * Caught in kvm_arch_vcpu_ioctl_run()
+ */
+ kvm_make_all_cpus_request(kvm, KVM_REQ_USERSPACE_KICK);
+
+ /*
+ * Use wr semaphore to wait for all vcpus to exit from KVM_RUN.
+ */
+ down_write(&memory_transaction);
+ up_write(&memory_transaction);
+ break;
+ }
+ case KVM_RESUME_ALL_KICKED_VCPUS: {
+ /* Remove all requests sent with KVM_KICK_ALL_RUNNING_VCPUS */
+ kvm_clear_all_cpus_request(kvm, KVM_REQ_USERSPACE_KICK);
+ break;
+ }
case KVM_SET_USER_MEMORY_REGION: {
struct kvm_userspace_memory_region kvm_userspace_mem;

--
2.31.1