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

[Christopher Covington <cov@codeaurora.org>]

On 11/04/2015 08:31 AM, Christoffer Dall wrote:
> 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.
>>>
>>> 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.

Is it not on by default since the following change?

http://git.qemu.org/?p=qemu.git;a=commitdiff;h=f022b8e95379b0433d13509706b66f38fc15dde8

Christopher Covington

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