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

[Leon Romanovsky]

On Wed, Jan 06, 2021 at 10:27:49AM +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.
>
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/linux-pci/87pn4nk7nn.fsf@xxxxxxxxxxxxxxxxxxxxxxx/
> Link: https://lore.kernel.org/linux-pci/877dqrnzr3.fsf@xxxxxxxxxxxxxxxxxxxxxxx/
> Link: https://lore.kernel.org/linux-pci/877dqqmc2h.fsf@xxxxxxxxxxxxxxxxxxxxxxx/
> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/pci/common.c       | 47 +++++++++++++++++++++++++++++++++++++
>  drivers/base/platform-msi.c |  8 +++++++
>  include/linux/msi.h         |  1 +
>  3 files changed, 56 insertions(+)
>
>
> Background:
> Learnt from the discussions in this thread:
>
> https://lore.kernel.org/linux-pci/160408357912.912050.17005584526266191420.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxx/
>
> The device IMS (Interrupt Message Storage) should not be enabled in any
> virtualization environments unless there is a HYPERCALL domain which
> makes the changes in the message store managed by the hypervisor.
>
> As the initial step, we allow the IMS to be enabled only if we are
> running on the bare metal. It's easy to enable IMS in the virtualization
> environments if above preconditions are met in the future.
>
> We ever thought about moving on_bare_metal() to a generic file so that
> it could be well maintained and used. But we need some suggestions about
> where to put it. Your comments are very appreciated.
>
> This patch is only for comments purpose. Please don't merge it. We will
> include it in the Intel IMS implementation later once we reach a
> consensus.
>
> Change log:
> v1->v2:
>  - v1:
>    https://lore.kernel.org/linux-pci/20201210004624.345282-1-baolu.lu@xxxxxxxxxxxxxxx/
>  - Rename probably_on_bare_metal() with on_bare_metal();
>  - Some vendors might use the same name for both bare metal and virtual
>    environment. Before we add vendor specific code to distinguish
>    between them, let's return false in on_bare_metal(). This won't
>    introduce any regression. The only impact is that the coming new
>    platform msi feature won't be supported until the vendor specific code
>    is provided.
>
> Best regards,
> baolu
>
> diff --git a/arch/x86/pci/common.c b/arch/x86/pci/common.c
> index 3507f456fcd0..963e0401f2b2 100644
> --- a/arch/x86/pci/common.c
> +++ b/arch/x86/pci/common.c
> @@ -724,3 +724,50 @@ struct pci_dev *pci_real_dma_dev(struct pci_dev *dev)
>  	return dev;
>  }
>  #endif
> +
> +/*
> + * 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.
> + *
> + * People might use the same vendor name for both bare metal and virtual
> + * environment. We can remove those names once we have vendor specific code to
> + * distinguish between them.
> + */
> +static const char * const vmm_vendor_name[] = {
> +	"QEMU", "Bochs", "KVM", "Xen", "VMware", "VMW", "VMware Inc.",
> +	"innotek GmbH", "Oracle Corporation", "Parallels", "BHYVE",
> +	"Microsoft Corporation", "Amazon EC2"
> +};

Maybe it is not concern at all, but this approach will make
forward/backward compatibility without kernel upgrade impossible.

Once QEMU (example) will have needed support, someone will need to remove
the QEMU from this array, rewrite on_bare_metal() because it is not bare
vs. virtual anymore and require kernel upgrade/downgrade every time QEMU
version is switched.

Plus need to update stable@ and distros.

I'm already feeling pain from the fields while they debug such code.

Am I missing it completely?

Thanks