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

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Date:   Wed, 16 Dec 2020 09:48:03 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     baolu.lu@...ux.intel.com, 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

Hi Bjorn,

On 12/11/20 2:57 AM, Bjorn Helgaas wrote:
> 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.

Agreed. we can use on_bare_metal() and add comments and kernel messages
to let users and developers know that we're not 100% sure. People should
help to make it more accurate by reporting exceptions.

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

Yes. This is a followup of previous discussion.

Best regards,
baolu