Re: [Xen-devel] [PATCH 7/7] xen/arm: Blacklist some sun7i UARTs

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2013-09-27 at 18:04 +0100, Andrew Cooper wrote:
> On 27/09/13 18:02, Julien Grall wrote:
> > On 09/22/2013 03:40 PM, Ian Campbell wrote:
> >> On Sat, 2013-09-21 at 21:28 +0100, Julien Grall wrote:
> >>> On 09/20/2013 10:41 PM, Ian Campbell wrote:
> >>>> On Fri, 2013-09-20 at 20:02 +0100, Julien Grall wrote:
> >>>>
> >>>>>> +    /*
> >>>>>> +     * These UARTs share a page with the Xen console UART, so we don't
> >>>>>> +     * want to map them through.
> >>>>>> +     */
> >>>>>> +    DT_MATCH_PATH("/soc@01c00000/serial@01c28000"),
> >>>>>> +    DT_MATCH_PATH("/soc@01c00000/serial@01c28400"),
> >>>>>> +    DT_MATCH_PATH("/soc@01c00000/serial@01c28800"),
> >>>>>> +    DT_MATCH_PATH("/soc@01c00000/serial@01c28c00"),
> >>>>> Can we blacklist all the UARTs via a DT_MATCH_COMPATIBLE? It's better
> >>>>> than relying on the path that can be changed easily in the device tree.
> >>>> There are other UARTS (at 0x1c29xxx) which could safely be exposed to
> >>>> dom0. Perhaps it is better to just blacklist the whole lot though.
> >>> These UARTs also share the same page, what prevents the user to use 
> >>> these UARTs?
> >> Nothing, but because they don't share a page with the xen UART they
> >> don't cause it to get exposed to dom0.
> > My question wasn't clear. What does prevent the user to use another UART
> > for Xen?
> >
> 
> Nothing "prevents" it.  That is the problem.  If several UARTs are on
> the same page, Xen cannot safely isolate its UART from dom0/domU.

He meant one of the other UARTs not on the same page as the primary one.
The answer is "not much".

However UART0 @ 0x1c28000 is the "debug" UART which is the one typically
wired up in a useful way on these platforms. The others are pin-muxed
with other functionality, such as MMC, LCD etc and are more often used
as such (I'm sure there will be exceptions).

That said Julien is probably right that the best solution is to just
blacklist all 8 UARTs.

Ian.


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