Re: [Qemu-devel] [RFC PATCH v4 5/5] target/ppc: support single stepping with KVM HV

[Fabiano Rosas]

Alexey Kardashevskiy <address@hidden> writes:

> Are you reposting this any time soon?

I have sent a v2 to the kernel side of it:

https://lore.kernel.org/kvm/address@hidden/

I'm depending on what we decide to do there. The core of this patchset
will not change, just the mechanism by which the feature is selected
(patch 2, kvm-all: Introduce kvm_set_singlestep).

> In meanwhile I hit a problem when I cannot step over the "stdu" instruction.
>
> I basically put this:
> stdu    r1,-368(r1)
>
> and "ni" in gdb does not stop on the next instruction which is quite
> confusing. Ideas?

Perhaps the next instruction is one that is not traced? See the programming
note at the end of section 6.5.15 in Book III.

Or maybe the step got preempted? You should see GDB messages indicating
changing threads right before or after the stdu. However that would only
happen with more than one VCPU and if 'show scheduler-locking' in GDB is
'off'. And even then, that should not cause any issues, but it is a more
complex scenario so there could be a bug in the code.

> On 20/03/2019 12:42, Alexey Kardashevskiy wrote:
>> 
>> 
>> On 20/03/2019 01:32, Fabiano Rosas wrote:
>>> Alexey Kardashevskiy <address@hidden> writes:
>>>
>>>> Looks good to me, does not break what already works. However I cannot
>>>> debug SLOF real mode and I am not sure why.
>>>>
>>>> (gdb) set endian big
>>>>
>>>> The target is assumed to be big endian
>>>> (gdb) b *0x3f00
>>>>
>>>> Breakpoint 2 at 0x3f00
>>>
>>> I think I'm missing the point here. Why 0x3f00?
>> 
>> Because I am stupid and did not realize that 0x3f00 is a relative offset
>> and 0x4000 is the correct address which works.
>> 
>> 
>> Reviewed-by: Alexey Kardashevskiy <address@hidden>
>> 
>> 
>>>
>>> (qemu) info roms
>>> addr=0000000000000000 size=0x0e22b8 mem=ram name="...qemu/slof.bin"         
>>>                       
>>> addr=0000000000400000 size=0x17976d0 mem=ram name="...vmlinux"
>>>
>>>
>>> $ objdump -d board-qemu/llfw/stage1.elf | grep "_start>"
>>> 0000000000000100 <__start>:
>>>      100:       48 00 3f 00     b       4000 <_start>
>>> 0000000000004000 <_start>:
>>>
>>>
>>> Thread 1 hit Breakpoint 3, _start () at startup.S:82
>>> (gdb) p/x $pc
>>> $1 = 0x4000
>>> (gdb) si
>>> (gdb) p/x $pc
>>> $3 = 0x4004
>>> (gdb) c
>>> Thread 1 hit Breakpoint 4, early_c_entry (start_addr=49056, fdt_addr=49024) 
>>> at stage2.c:202
>>> (gdb) p/x $pc
>>> $4 = 0x4d18
>>>
>>