Re: [Qemu-devel] [PATCH] memory: Use standard log messages for unassigned memory read / write

[Stefan Weil]

Am 08.09.2016 um 11:37 schrieb Paolo Bonzini:
> On 08/09/2016 07:43, Stefan Weil wrote:
> > Am 07.09.2016 um 23:29 schrieb Peter Maydell:
> > > On 7 September 2016 at 20:13, Stefan Weil <address@hidden> wrote:
> > > > The old log messages are implemented by conditional compilation
> > > > and not available by default.
> > > >
> > > > The new log messages can be enabled either by a command line option
> > > > (-d unimp) or in the QEMU monitor (log unimp).
> > > >
> > > > Signed-off-by: Stefan Weil <address@hidden>
> > > > ---
> > > >
> > > > The new code is very useful when implementing new platforms or
> > > > looking for problems with existing platforms which are only
> > > > partially emulated (I use it for Raspberry Pi).
> > > >
> > > > target-sparc/ldst_helper.c also uses DEBUG_UNASSIGNED
> > > > and could get similar code.
> > > >
> > > > Regards,
> > > > Stefan
> > > >
> > > >
> > > >  memory.c | 12 ++++--------
> > > >  1 file changed, 4 insertions(+), 8 deletions(-)
> >
> > [...]
> >
> > > > -#ifdef DEBUG_UNASSIGNED
> > > > -    printf("Unassigned mem write " TARGET_FMT_plx " =
> > > > 0x%"PRIx64"\n", addr, val);
> > > > -#endif
> > > > +    qemu_log_mask(LOG_UNIMP, "%s " TARGET_FMT_plx " = 0x%" PRIx64 "\n",
> > > > +                  __func__, addr, val);
> > >
> > > Maybe this should be LOG_GUEST_ERROR rather than LOG_UNIMP ?
> >
> > My first code used LOG_GUEST_ERROR. Of course a guest can try to access
> > memory which is not available for the real hardware.
> >
> > For my test object (Raspberry Pi), all unassigned memory accesses were
> > not guest errors but unimplemented emulation in QEMU – that's why I
> > changed from LOG_GUEST_ERROR to LOG_UNIMP.
> >
> > A short test with i386 / x86_64 emulation also detected unassigned
> > memory accesses (from BIOS) caused by an unimplemented debug device, so
> > that was also not a guest error.
>
> FWIW, the debug device is optional (-device isa-debugcon), not
> unimplemented.  Similarly it's common, or at least it was before ACPI
> was introduced, to poke at known ISA ports to detect legacy devices;
> it's neither unimplemented nor a guest error in that case.
>
> Perhaps it should just be a tracepoint?
>
> Paolo

Yes, especially for x86 there are several devices which are optional
(e. g. serial and parallel ports). I also found the isa-debugcon device
when I searched for the reason of the unassigned memory access. Maybe 
isa-debugcon would deserve better documentation.

Regarding using qemu_log_mask or using tracepoints 
(trace_unassigned_mem_read, trace_unassigned_mem_write?): Both 
alternatives are fine for me. I just want something which does not need 
a new build with conditional compilation. It should be possible to 
enable the log / trace message at any time. I can also prepare a new 
patch with tracepoints. Any more feedback which variant is preferred?

Stefan