Re: [RFC V2] xen: interface: introduce pvclk interface

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2016-01-21 at 16:59 +0800, Peng Fan wrote:
> 
> To platform device of ARM, hypervisor is responsible for the mapping
> between machine address and guest physical address, also responsible
> for the irq mapping.
> 
> But to embedded ARM SoC, the hardware clk IP is handled in Dom0.

Arguably Xen ought to be the one to do this, but we have determined
(rightly, I think) that doing so for the entirely clk tree of an SoC would
involve importing an unmanageable amount of code into Xen[0].

In the meantime we defer this to Dom0.

I wonder though if it would be possible to manage the clocks for a
passthrough device from the toolstack, i.e. is there a sysfs node where one
can say "keep the clock for this device enabled (at xMhz) even though you
think the device is unused"?

If so (or if it can be easily added) then the toolstack would just need to
manage that value as part of the passthrough process rather than having the
frontend do it via a PV protocol.

Ian.

[0] I'd like at some point for Xen to gain sufficient knowledge of clock IP
to minimally control things like the CPU and DRAM clocks etc, but that
needn't imply full clock tree support and would hopefully be a manageable
amount of code, which would (hopefully) mostly be about trapping and
emulating writes to one or two clock controllers per platform and
arbitrating a small set of bits (while allowing dom0 to control the
others). This is at least a medium if not long term idea though, and for
all I know it might turn out to be unworkable in practice.


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