linux-kernel - [PATCH 4/4] KVM: use signals to abort enter

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Date:   Sat, 22 Oct 2022 11:48:19 -0400
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>
Subject: [PATCH 4/4] KVM: use signals to abort enter_guest/blocking and retry

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@...hat.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