linux-kernel - RE: [PATCH] x86/hyperv: Enable 15-bit APIC ID if the hypervisor supports it

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Message-ID: <MWHPR21MB1546EF053D64DEBA1A586E63BF110@MWHPR21MB1546.namprd21.prod.outlook.com>
Date:   Tue, 3 Nov 2020 09:46:19 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        vkuznets <vkuznets@...hat.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>
Subject: RE: [PATCH] x86/hyperv: Enable 15-bit APIC ID if the hypervisor
 supports it

> From: David Woodhouse <dwmw2@...radead.org>
> Sent: Tuesday, November 3, 2020 12:03 AM
> > +/*
> > + * If ms_hyperv_msi_ext_dest_id() returns true,
> > hyperv_prepare_irq_remapping()
> > + * returns -ENODEV and the Hyper-V IOMMU driver is not used; instead, the
> > + * generic support of the 15-bit APIC ID is used: see
> > __irq_msi_compose_msg().
> > + *
> > + * Note: For a VM on Hyper-V, no emulated legacy device supports PCI
> MSI/MSI-X,
> > + * and PCI MSI/MSI-X only come from the assigned physical PCIe device, and
> the
> > + * PCI MSI/MSI-X interrupts are handled by the pci-hyperv driver. Here
> despite
> > + * the word "msi" in the name "msi_ext_dest_id", actually the callback only
> > + * affects how IOAPIC interrupts are routed.
> > + */
> 
> I named it like that on purpose to make the point that the I/OAPIC is
> just a device for turning line interrupts into MSIs. Some VMMs, just
> like real hardware, really do implement their I/OAPIC emulation that
> way. It makes a lot of sense to do so if you support interrupt
> remapping.

I totally agree.
 
> FWIW I might have phrased your last paragraph in that comment as
> 
>   Note: for a VM on Hyper-V, the I/OAPIC is the only device which
>   (logically) generates MSIs directly to the system APIC irq domain.
>   There is no HPET, and PCI MSI/MSI-X interrupts are remapped by the
>   pci-hyperv host bridge.

I agree. This version is much better.
 
> But don't bother to change it; I think I've made my point quite well
> enough with https://git.kernel.org/tip/tip/c/5d5a97133 :)
> 
> --
> dwmw2

Hi David,
This patch has been in the x86/apic branch (with a line missing in the commit
log). If possible, I hope tglx can help make this change you suggested, and add
the missing line in the commit log. :-)

Thanks,
-- Dexuan