KVM: KVM: Use cpumask_available() to check for NULL cpumask when kicking vCPUs

Check for a NULL cpumask_var_t when kicking multiple vCPUs via
cpumask_available(), which performs a !NULL check 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.

Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210827092516.1027264-3-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 490c8cb8cc8d0bcba74fb7810e9ab951c7c48d35..e95e7a9e4d53fc700e4aefab7a6ddf708a7cb41e 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -235,9 +235,13 @@ static void ack_flush(void *_completed)
 {
 }
 
-static inline bool kvm_kick_many_cpus(const struct cpumask *cpus, bool wait)
+static inline bool kvm_kick_many_cpus(cpumask_var_t tmp, bool wait)
 {
-       if (unlikely(!cpus))
+       const struct cpumask *cpus;
+
+       if (likely(cpumask_available(tmp)))
+               cpus = tmp;
+       else
                cpus = cpu_online_mask;
 
        if (cpumask_empty(cpus))
@@ -267,6 +271,14 @@ 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 "unavailable" 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 (!cpumask_available(tmp))
+                       continue;
+
                /*
                 * Note, the vCPU could get migrated to a different pCPU at any
                 * point after kvm_request_needs_ipi(), which could result in
@@ -278,7 +290,7 @@ 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);