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

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Date:   Mon, 23 Aug 2021 09:54:16 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Venkatesh Srinivas <venkateshs@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 2/2] KVM: Guard cpusmask NULL check with
 CONFIG_CPUMASK_OFFSTACK

Sean Christopherson <seanjc@...gle.com> writes:

> 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);

In case MM people don't like us poking into CONFIG_CPUMASK_OFFSTACK
details we can probably get away with a comment. Otherwise

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly