linux-kernel - [PATCH 2/2] KVM: Guard cpusmask NULL check with CONFIG_CPUMASK

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Message-Id: <20210821000501.375978-3-seanjc@google.com>
Date:   Fri, 20 Aug 2021 17:05:01 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Venkatesh Srinivas <venkateshs@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH 2/2] KVM: Guard cpusmask NULL check with CONFIG_CPUMASK_OFFSTACK

Check for a NULL cpumask_var_t when kicking multiple vCPUs if and only if
cpumasks are configured to be allocated off-stack.  This is a meaningless
optimization, e.g. avoids a TEST+Jcc and TEST+CMOV on x86, but more
importantly helps document that the NULL check is necessary even though
all callers pass in a local variable.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 virt/kvm/kvm_main.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 786b914db98f..82c5280dd5ce 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -247,7 +247,7 @@ static void ack_flush(void *_completed)
 
 static inline bool kvm_kick_many_cpus(const struct cpumask *cpus, bool wait)
 {
-	if (unlikely(!cpus))
+	if (IS_ENABLED(CONFIG_CPUMASK_OFFSTACK) && unlikely(!cpus))
 		cpus = cpu_online_mask;
 
 	if (cpumask_empty(cpus))
@@ -277,6 +277,14 @@ bool kvm_make_vcpus_request_mask(struct kvm *kvm, unsigned int req,
 		if (!(req & KVM_REQUEST_NO_WAKEUP) && kvm_vcpu_wake_up(vcpu))
 			continue;
 
+		/*
+		 * tmp can be NULL if cpumasks are allocated off stack, as
+		 * allocation of the mask is deliberately not fatal and is
+		 * handled by falling back to kicking all online CPUs.
+		 */
+		if (IS_ENABLED(CONFIG_CPUMASK_OFFSTACK) && !tmp)
+			continue;
+
 		/*
 		 * Note, the vCPU could get migrated to a different pCPU at any
 		 * point after kvm_request_needs_ipi(), which could result in
@@ -288,7 +296,7 @@ bool kvm_make_vcpus_request_mask(struct kvm *kvm, unsigned int req,
 		 * were reading SPTEs _before_ any changes were finalized.  See
 		 * kvm_vcpu_kick() for more details on handling requests.
 		 */
-		if (tmp != NULL && kvm_request_needs_ipi(vcpu, req)) {
+		if (kvm_request_needs_ipi(vcpu, req)) {
 			cpu = READ_ONCE(vcpu->cpu);
 			if (cpu != -1 && cpu != me)
 				__cpumask_set_cpu(cpu, tmp);
-- 
2.33.0.rc2.250.ged5fa647cd-goog