Re: [RFC PATCH v2 1/1] platform-msi: Add platform check for subdevice irq domain

[Leon Romanovsky]

On Thu, Jan 07, 2021 at 06:55:16AM +0000, Tian, Kevin wrote:
> > From: Leon Romanovsky <leon@xxxxxxxxxx>
> > Sent: Thursday, January 7, 2021 2:09 PM
> >
> > On Thu, Jan 07, 2021 at 02:04:29AM +0000, Tian, Kevin wrote:
> > > > From: Leon Romanovsky <leon@xxxxxxxxxx>
> > > > Sent: Thursday, January 7, 2021 12:02 AM
> > > >
> > > > On Wed, Jan 06, 2021 at 11:23:39AM -0400, Jason Gunthorpe wrote:
> > > > > On Wed, Jan 06, 2021 at 12:40:17PM +0200, Leon Romanovsky wrote:
> > > > >
> > > > > > I asked what will you do when QEMU will gain needed functionality?
> > > > > > Will you remove QEMU from this list? If yes, how such "new" kernel
> > will
> > > > > > work on old QEMU versions?
> > > > >
> > > > > The needed functionality is some VMM hypercall, so presumably new
> > > > > kernels that support calling this hypercall will be able to discover
> > > > > if the VMM hypercall exists and if so superceed this entire check.
> > > >
> > > > Let's not speculate, do we have well-known path?
> > > > Will such patch be taken to stable@/distros?
> > > >
> > >
> > > There are two functions introduced in this patch. One is to detect whether
> > > running on bare metal or in a virtual machine. The other is for deciding
> > > whether the platform supports ims. Currently the two are identical because
> > > ims is supported only on bare metal at current stage. In the future it will
> > look
> > > like below when ims can be enabled in a VM:
> > >
> > > bool arch_support_pci_device_ims(struct pci_dev *pdev)
> > > {
> > > 	return on_bare_metal() || hypercall_irq_domain_supported();
> > > }
> > >
> > > The VMM vendor list is for on_bare_metal, and suppose a vendor will
> > > never be removed once being added to the list since the fact of running
> > > in a VM never changes, regardless of whether this hypervisor supports
> > > extra VMM hypercalls.
> >
> > This is what I imagined, this list will be forever, and this worries me.
> >
> > I don't know if it is true or not, but guess that at least Oracle and
> > Microsoft bare metal devices and VMs will have same DMI_SYS_VENDOR.
>
> It's true. David Woodhouse also said it's the case for Amazon EC2 instances.
>
> >
> > It means that this on_bare_metal() function won't work reliably in many
> > cases. Also being part of include/linux/msi.h, at some point of time,
> > this function will be picked by the users outside for the non-IMS cases.
> >
> > I didn't even mention custom forks of QEMU which are prohibited to change
> > DMI_SYS_VENDOR and private clouds with custom solutions.
>
> In this case the private QEMU forks are encouraged to set CPUID (X86_
> FEATURE_HYPERVISOR) if they do plan to adopt a different vendor name.

Does QEMU set this bit when it runs in host-passthrough CPU model?

>
> >
> > The current array makes DMI_SYS_VENDOR interface as some sort of ABI. If
> > in the future,
> > the QEMU will decide to use more hipster name, for example "qEmU", this
> > function
> > won't work.
> >
> > I'm aware that DMI_SYS_VENDOR is used heavily in the kernel code and
> > various names for the same company are good example how not reliable it.
> >
> > The most hilarious example is "Dell/Dell Inc./Dell Inc/Dell Computer
> > Corporation/Dell Computer",
> > but other companies are not far from them.
> >
> > Luckily enough, this identification is used for hardware product that
> > was released to the market and their name will be stable for that
> > specific model. It is not the case here where we need to ensure future
> > compatibility too (old kernel on new VM emulator).
> >
> > I'm not in position to say yes or no to this patch and don't have plans to do it.
> > Just expressing my feeling that this solution is too hacky for my taste.
> >
>
> I agree with your worries and solely relying on DMI_SYS_VENDOR is
> definitely too hacky. In previous discussions with Thomas there is no
> elegant way to handle this situation. It has to be a heuristic approach.
> First we hope the CPUID bit is set properly in most cases thus is checked
> first. Then other heuristics can be made for the remaining cases. DMI_
> SYS_VENDOR is the first hint and more can be added later. For example,
> when IOMMU is present there is vendor specific way to detect whether
> it's real or virtual. Dave also mentioned some BIOS flag to indicate a
> virtual machine. Now probably the real question here is whether people
> are OK with CPUID+DMI_SYS_VENDOR combo check for now (and grow
> it later) or prefer to having all identified heuristics so far in-place together...

IMHO, it should be as much as possible close to the end result.

Thanks

>
> Thanks
> Kevin