Re: [PATCH 2/2] KVM: Guard cpusmask NULL check with CONFIG_CPUMASK_OFFSTACK

[Sean Christopherson <seanjc@google.com>]

On Wed, Aug 25, 2021, Lai Jiangshan wrote:
> On Sat, Aug 21, 2021 at 8:09 AM Sean Christopherson <seanjc@google.com> wrote:
> > @@ -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;
> > +
> 
> Hello, Sean
> 
> I don't think it is a good idea to reinvent the cpumask_available().

Using cpumask_available() is waaaay better, thanks!

Vitaly / Paolo, take this one instead?

From deff3e168c0612a2947d1ef29e488282631a788c Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Fri, 20 Aug 2021 13:36:21 -0700
Subject: [PATCH] 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>
---
 virt/kvm/kvm_main.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 786b914db98f..2082aceffbf6 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -245,9 +245,13 @@ 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))
@@ -277,6 +281,14 @@ 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
@@ -288,7 +300,7 @@ 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);
--