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From:   Michael Kelley <mikelley@microsoft.com>
To:     "lantianyu1986@gmail.com" <lantianyu1986@gmail.com>
CC:     Tianyu Lan <Tianyu.Lan@microsoft.com>,
        "joro@8bytes.org" <joro@8bytes.org>,
        "mchehab+samsung@kernel.org" <mchehab+samsung@kernel.org>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "nicolas.ferre@microchip.com" <nicolas.ferre@microchip.com>,
        "arnd@arndb.de" <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        KY Srinivasan <kys@microsoft.com>,
        vkuznets <vkuznets@redhat.com>,
        "alex.williamson@redhat.com" <alex.williamson@redhat.com>,
        "sashal@kernel.org" <sashal@kernel.org>,
        "dan.carpenter@oracle.com" <dan.carpenter@oracle.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
Subject: RE: [PATCH V4 2/3] HYPERV/IOMMU: Add Hyper-V stub IOMMU driver
Thread-Topic: [PATCH V4 2/3] HYPERV/IOMMU: Add Hyper-V stub IOMMU driver
Thread-Index: AQHUwhUChdXB9clmF0W3TeUetJI4UqXqj7jg
Date:   Thu, 21 Feb 2019 17:27:37 +0000
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From: lantianyu1986@gmail.com <lantianyu1986@gmail.com> Sent: Monday, Febru=
ary 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 =3D min_t(unsigned int, num_possible_cpus(), 255); i >=3D 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