Re: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

Raj, Ashok
2020-11-11T16:09:40Z

On Wed, Nov 11, 2020 at 03:41:59PM +0000, Christoph Hellwig wrote:
> On Sun, Nov 08, 2020 at 07:36:34PM +0000, David Woodhouse wrote:
> > So it does look like we're going to need a hypercall interface to
> > compose an MSI message on behalf of the guest, for IMS to use. In fact
> > PCI devices assigned to a guest could use that too, and then we'd only
> > need to trap-and-remap any attempt to write a Compatibility Format MSI
> > to the device's MSI table, while letting Remappable Format messages get
> > written directly.
> >
> > We'd also need a way for an OS running on bare metal to *know* that
> > it's on bare metal and can just compose MSI messages for itself. Since
> > we do expect bare metal to have an IOMMU, perhaps that is just a
> > feature flag on the IOMMU?
>
> Have the platform firmware advertise if it needs native or virtualized
> IMS handling. If it advertises neither don't support IMS?

The platform hint can be easily accomplished via DMAR table flags. We could
have an IMS_OPTOUT(similart to x2apic optout flag) flag, when 0 its native
and IMS is supported.

When vIOMMU is presented to guest, virtual DMAR table will have this flag
set to 1. Indicates to GuestOS, native IMS isn't supported.