Re: [PATCH 0/3] KVM: x86: KVM_MEM_PCI_HOLE memory

[Michael S. Tsirkin]

On Thu, Aug 06, 2020 at 01:39:09PM +0200, Vitaly Kuznetsov wrote:
> "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes:
> 
> > About the feature bit, I am not sure why it's really needed. A single
> > mmio access is cheaper than two io accesses anyway, right? So it makes
> > sense for a kvm guest whether host has this feature or not.
> > We need to be careful and limit to a specific QEMU implementation
> > to avoid tripping up bugs, but it seems more appropriate to
> > check it using pci host IDs.
> 
> Right, it's just that "running on KVM" is too coarse grained, we just
> need a way to somehow distinguish between "known/good" and
> "unknown/buggy" configurations.

Basically it's not KVM, it's QEMU that is known good.  QEMU vendor id in
the pci host seems like a reasonable way to detect that. If someone
reuses QEMU ID - I guess they better behave just like QEMU :)

I also proposed only limiting this to register 0 (device id),
will make it very unlikely this can break accidentally ...

> -- 
> Vitaly