Re: [PATCH] KVM/arm: kernel low level debug support for ARM32 virtual platforms

[Christoffer Dall <christoffer.dall@linaro.org>]

On Tue, Nov 03, 2015 at 01:39:44PM -0600, Rob Herring wrote:
> On Tue, Nov 3, 2015 at 1:17 PM, Mario Smarduch <m.smarduch@samsung.com> wrote:
> >
> >
> > On 11/3/2015 9:55 AM, Will Deacon wrote:
> >> On Tue, Nov 03, 2015 at 09:44:52AM -0800, Mario Smarduch wrote:
> >>> On 11/3/2015 8:33 AM, Christopher Covington wrote:
> >>>> On 11/02/2015 06:51 PM, Mario Smarduch wrote:
> >>>>>    this is a re-post from couple weeks ago, please take time to review this
> >>>>> simple patch which simplifies DEBUG_LL and prevents kernel crash on virtual
> >>>>> platforms.
> >>>>>
> >>>>> Before this patch DEBUG_LL for 'dummy virtual machine':
> >>>>>
> >>>>> ( ) Kernel low-level debugging via EmbeddedICE DCC channel
> >>>>> ( ) Kernel low-level debug output via semihosting I/O
> >>>>> ( ) Kernel low-level debugging via 8250 UART
> >>>>> ( ) Kernel low-level debugging via ARM Ltd PL01x Primecell
> >>>>>
> >>>>> In summary if debug uart is not emulated kernel crashes.
> >>>>> And once you pass that hurdle, uart physical/virtual addresses are unknown.
> >>>>> DEBUG_LL comes in handy on many occasions and should be somewhat
> >>>>> intuitive to use like it is for physical platforms. For virtual platforms
> >>>>> user may start daubting the host and get into a bigger mess.
> >>>>>
> >>>>> After this patch is applied user gets:
> >>>>>
> >>>>> (X) Kernel low-level debugging on QEMU Virtual Platform
> >>>>> ( ) Kernel low-level debugging on Kvmtool Virtual Platform
> >>>>>    ..... above repeated ....
> >>>>>
> >>>>> The virtual addresses selected follow arm reference models, high in vmalloc
> >>>>> section with high mem enabled and guest running with >= 1GB of memory. The
> >>>>> offset is leftover from arm reference models.
> >>>>
> >>>> Which model? It doesn't appear to match the vexpress AEM/RTSM/FVP/whatever
> >>>> which used 0x1c090000 for UART0.
> >>>
> >>> I recall QEMU virt model had it's own physical address map, for sure I saw the
> >>> virtio-mmio regions assigned in some ARM document. Peter would you know?
> >>>
> >>> As far as kvmtool I'm not sure, currently PC1 COM1 port is used? Andre will that
> >>> stay fixed?
> >>
> >> We make absolutely no guarantees about the memory map provided by kvmtool.
> >>
> >> Will
> >>
> >
> > If that's also the case for qemu, then I guess the best you can do is find a way
> > to dump the device tree. Find the uart, physical address and try figure out the
> > virtual address.
> >
> > Pretty involved, hoped for something more automated since that's a handy feature.
> 
> You really only need LL_DEBUG now if you are debugging very early code
> before memory is setup and/or bad memory. Use earlycon instead which
> should already be supported both via the pl011 or semihosting. I used
> it with QEMU semihosting support.
> 
Then we should really document how to use that with qemu's virt platform
and kvmtool's platform on both 32-bit and 64-bit so that users can
easily figure out what they're doing wrong when they get no output.

In practice, the address for the pl011 is quite unlikely to change, I
dare speculate, so that documentation shouldn't need frequent updating.

-Christoffer