linux-kernel - Re: [PATCH v2 1/2] KVM: X86: Implement PV IPI in linux guest

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Date:   Mon, 2 Jul 2018 13:32:50 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v2 1/2] KVM: X86: Implement PV IPI in linux guest

On 02/07/2018 12:14, Wanpeng Li wrote:
> +	unsigned long flags, ipi_bitmap_low = 0, ipi_bitmap_high = 0, icr = 0;
> +	int cpu, apic_id, ret = 1;
> +
> +	if (cpumask_empty(mask))
> +		return true;
> +
> +	local_irq_save(flags);
> +
> +	for_each_cpu(cpu, mask) {
> +		apic_id = per_cpu(x86_cpu_to_apicid, cpu);
> +		if (apic_id < BITS_PER_LONG)
> +			__set_bit(apic_id, &ipi_bitmap_low);
> +		else if (apic_id < 2 * BITS_PER_LONG)
> +			__set_bit(apic_id - BITS_PER_LONG, &ipi_bitmap_high);
> +		else
> +			goto ipi_mask_done;
> +	}

CPU masks are themselves bitmaps made of longs, so you should be able to
avoid the loop here.

> +static void kvm_send_ipi_mask(const struct cpumask *mask, int vector)
> +{
> +	if (!__send_ipi_mask(mask, vector))
> +		orig_apic.send_IPI_mask(mask, vector);
> +}
> +
> +static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
> +{
> +	unsigned int this_cpu = smp_processor_id();
> +	struct cpumask new_mask;
> +	const struct cpumask *local_mask;
> +
> +	cpumask_copy(&new_mask, mask);
> +	cpumask_clear_cpu(this_cpu, &new_mask);
> +	local_mask = &new_mask;
> +	if (!__send_ipi_mask(local_mask, vector))
> +		orig_apic.send_IPI_mask_allbutself(mask, vector);
> +}

Likewise, here it should be possible to check the highest bit in the
mask before copying it.

Paolo