linux-kernel - Re: [RFC PATCH 1/1] platform-msi: Add platform check for subdevice irq domain

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Date:   Thu, 10 Dec 2020 12:57:38 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     tglx@...utronix.de, ashok.raj@...el.com, kevin.tian@...el.com,
        dave.jiang@...el.com, megha.dey@...el.com,
        alex.williamson@...hat.com, bhelgaas@...gle.com,
        dan.j.williams@...el.com, dmaengine@...r.kernel.org,
        eric.auger@...hat.com, jacob.jun.pan@...el.com, jgg@...lanox.com,
        jing.lin@...el.com, kvm@...r.kernel.org, kwankhede@...dia.com,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        maz@...nel.org, mona.hossain@...el.com, netanelg@...lanox.com,
        parav@...lanox.com, pbonzini@...hat.com, rafael@...nel.org,
        samuel.ortiz@...el.com, sanjay.k.kumar@...el.com,
        shahafs@...lanox.com, tony.luck@...el.com, vkoul@...nel.org,
        yan.y.zhao@...ux.intel.com, yi.l.liu@...el.com
Subject: Re: [RFC PATCH 1/1] platform-msi: Add platform check for subdevice
 irq domain

On Thu, Dec 10, 2020 at 08:46:24AM +0800, Lu Baolu wrote:
> The pci_subdevice_msi_create_irq_domain() should fail if the underlying
> platform is not able to support IMS (Interrupt Message Storage). Otherwise,
> the isolation of interrupt is not guaranteed.
> 
> For x86, IMS is only supported on bare metal for now. We could enable it
> in the virtualization environments in the future if interrupt HYPERCALL
> domain is supported or the hardware has the capability of interrupt
> isolation for subdevices.

> + * We want to figure out which context we are running in. But the hardware
> + * does not introduce a reliable way (instruction, CPUID leaf, MSR, whatever)
> + * which can be manipulated by the VMM to let the OS figure out where it runs.
> + * So we go with the below probably_on_bare_metal() function as a replacement
> + * for definitely_on_bare_metal() to go forward only for the very simple reason
> + * that this is the only option we have.
> + */
> +static const char * const possible_vmm_vendor_name[] = {
> +	"QEMU", "Bochs", "KVM", "Xen", "VMware", "VMW", "VMware Inc.",
> +	"innotek GmbH", "Oracle Corporation", "Parallels", "BHYVE",
> +	"Microsoft Corporation"
> +};
> +
> +static bool probably_on_bare_metal(void)

What is the point of a function called probably_on_bare_metal()?
*Probably*?  The caller can't really do anything with the fact that
we're not 100% sure this gives the correct answer.  Just call it
"on_bare_metal()" or something and accept the fact that it might be
wrong sometimes.

This patch goes with IMS support, which somebody else is handling, so
I assume you don't need anything from the PCI side.