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

[Thomas Gleixner]

On Wed, Nov 11 2020 at 08:09, Ashok Raj wrote:
> 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.

These opt-out bits suck by definition. It comes all back to the fact
that the whole virt thing didn't have a hardware defined way to tell
that the OS runs in a VM and not on bare metal. It wouldn't have been
rocket science to do so.

And because that does not exist, we need magic opt-out bits for every
other piece of functionality which gets added. Can we please stop this
and provide a well defined way to tell the OS whether it runs on bare
metal or not?

The point is that you really want opt-in bits so that decisions come
down to

     if (!virt || virt->supports_X)

which is the obvious sane and safe logic. But sure, why am I asking for
sane and safe in the context of virtualization?

Thanks,

        tglx