linux-kernel - RE: [PATCH V4 2/3] HYPERV/IOMMU: Add Hyper-V stub IOMMU driver

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Date:   Thu, 21 Feb 2019 17:27:37 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     "lantianyu1986@...il.com" <lantianyu1986@...il.com>
CC:     Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "mchehab+samsung@...nel.org" <mchehab+samsung@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "nicolas.ferre@...rochip.com" <nicolas.ferre@...rochip.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        KY Srinivasan <kys@...rosoft.com>,
        vkuznets <vkuznets@...hat.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "sashal@...nel.org" <sashal@...nel.org>,
        "dan.carpenter@...cle.com" <dan.carpenter@...cle.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Subject: RE: [PATCH V4 2/3] HYPERV/IOMMU: Add Hyper-V stub IOMMU driver

From: lantianyu1986@...il.com <lantianyu1986@...il.com> Sent: Monday, February 11, 2019 6:20 AM
> +	/*
> +	 * Hyper-V doesn't provide irq remapping function for
> +	 * IO-APIC and so IO-APIC only accepts 8-bit APIC ID.
> +	 * Cpu's APIC ID is read from ACPI MADT table and APIC IDs
> +	 * in the MADT table on Hyper-v are sorted monotonic increasingly.
> +	 * APIC ID reflects cpu topology. There maybe some APIC ID
> +	 * gaps when cpu number in a socket is not power of two. Prepare
> +	 * max cpu affinity for IOAPIC irqs. Scan cpu 0-255 and set cpu
> +	 * into ioapic_max_cpumask if its APIC ID is less than 256.
> +	 */
> +	for (i = min_t(unsigned int, num_possible_cpus(), 255); i >= 0; i--)

The above isn't quite right.  For example, if num_possible_cpus() is 8,
then the loop will be executed 9 times, for values 8 down through 0.
It should be executed for values 7 down through 0.

> +		if (cpu_physical_id(i) < 256)
> +			cpumask_set_cpu(i, &ioapic_max_cpumask);
> +
> +	return 0;
> +}

Michael