netdev - Re: MDIO Debug Interface

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Message-Id: <C42U7ICFS9TP.3PIIHGICUXOC4@wkz-x280>
Date:   Fri, 10 Jul 2020 11:30:21 +0200
From:   "Tobias Waldekranz" <tobias@...dekranz.com>
To:     "Vladimir Oltean" <olteanv@...il.com>,
        "Andrew Lunn" <andrew@...n.ch>
Cc:     "Tobias Waldekranz" <tobias@...dekranz.com>,
        <netdev@...r.kernel.org>, <f.fainelli@...il.com>,
        <hkallweit1@...il.com>
Subject: 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:
> > > 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.