Re: MDIO Debug Interface

On Fri Jul 10, 2020 at 4:33 AM CEST, Vladimir Oltean wrote:

On Fri, Jul 10, 2020 at 01:20:35AM +0200, Andrew Lunn wrote:

quoted

quoted

Virtualization is a reasonable use case in my opinion and it would
need something like this, for the guest kernel to have access to its
PHY.

And i would like a better understanding of this use case. It seems odd
you are putting the driver for a PHY in the VM. Is the MAC driver also
in the VM? As you said, VM context switches are expensive, so it would
seem to make more sense to let the host drive the hardware, which it
can do without all these context switches, and export a much simpler
and efficient API to the VM.

    Andrew

The MAC driver is also in the VM, yes, and the VM can be given
pass-through access to the MAC register map. But the PHY is on a shared
bus. It is not desirable to give the VM access to the entire MDIO
controller's register map, for a number of reasons which I'm sure I
don't need to enumerate. So QEMU should be instructed which PHY should
each network interface use and on which bus, and it should fix-up the
device tree of the guest with a phy-handle to a PHY on a
para-virtualized MDIO bus, guest-side driver of which is going to be
accessing the real MDIO bus through this UAPI which we're talking about.
Then the host-side MDIO driver can ensure serialization of MDIO
accesses, etc etc.

It's been a while since I looked at this, so I may be lacking some of
the technical details, and brushing them up is definitely not something
for today.

-Vladimir

Have you considered the case where the PHY is only a slice of a quad-
or octal PHY?

I know that on at least some of these chips, all slices have access to
global registers that can have destructive effects on neighboring
slices. That could make it very difficult to implement a secure
solution using this architecture.