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

[Lu Baolu]

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