Re: [Qemu-devel] [PATCH v2 6/7] scripts/dump-guest-memory.py: add vmcoreinfo

["Marc-André Lureau" <marcandre.lureau@redhat.com>]

Hi

----- Original Message -----
> On 07/06/17 12:16, Marc-André Lureau wrote:
> > Add vmcoreinfo ELF note if vmcoreinfo device is ready.
> > 
> > To help the python script, add a little global vmcoreinfo_gdb
> > structure, that is populated with vmcoreinfo_gdb_update().
> > 
> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > ---
> >  scripts/dump-guest-memory.py | 40 ++++++++++++++++++++++++++++++++++++++++
> >  hw/acpi/vmcoreinfo.c         |  3 +++
> >  2 files changed, 43 insertions(+)
> > 
> > diff --git a/scripts/dump-guest-memory.py b/scripts/dump-guest-memory.py
> > index f7c6635f15..2dd2ed6983 100644
> > --- a/scripts/dump-guest-memory.py
> > +++ b/scripts/dump-guest-memory.py
> > @@ -14,6 +14,7 @@ the COPYING file in the top-level directory.
> >  """
> >  
> >  import ctypes
> > +import struct
> >  
> >  UINTPTR_T = gdb.lookup_type("uintptr_t")
> >  
> > @@ -120,6 +121,20 @@ class ELF(object):
> >          self.segments[0].p_filesz += ctypes.sizeof(note)
> >          self.segments[0].p_memsz += ctypes.sizeof(note)
> >  
> > +
> > +    def add_vmcoreinfo_note(self, vmcoreinfo):
> > +        """Adds a vmcoreinfo note to the ELF dump."""
> > +        chead = type(get_arch_note(self.endianness, 0, 0))
> > +        header =
> > chead.from_buffer_copy(vmcoreinfo[0:ctypes.sizeof(chead)])
> 
> Maybe it's obvious to others, but I would have been helped a lot if a
> comment had explained that you are creating a fake note (with 0 desc
> size and 0 name size) to figure out the size of the note header. And
> then you copy that many bytes out of the vmcoreinfo ELF note.
> 

ok

> > +        note = get_arch_note(self.endianness,
> > +                             header.n_namesz - 1, header.n_descsz)
> 
> Why the -1?

because get_arch_note() adds + 1 (supposedly for the ending \0, see also add_note())

> 
> ... I think I'm giving up here for this method. My python is weak and I
> can't follow this too well. Please add some comments.

I simplified a bit the code now, I was also discovering some ctype usage.

> More comments below:
> 
> > +        ctypes.memmove(ctypes.pointer(note), vmcoreinfo,
> > ctypes.sizeof(note))
> > +        header_size = ctypes.sizeof(note) - header.n_descsz
> > +
> > +        self.notes.append(note)
> > +        self.segments[0].p_filesz += ctypes.sizeof(note)
> > +        self.segments[0].p_memsz += ctypes.sizeof(note)
> > +
> >      def add_segment(self, p_type, p_paddr, p_size):
> >          """Adds a segment to the elf."""
> >  
> > @@ -505,6 +520,30 @@ shape and this command should mostly work."""
> >                  cur += chunk_size
> >                  left -= chunk_size
> >  
> > +    def phys_memory_read(self, addr, size):
> > +        qemu_core = gdb.inferiors()[0]
> > +        for block in self.guest_phys_blocks:
> > +            if block["target_start"] <= addr < block["target_end"]:
> 
> Although I don't expect a single read to straddle phys-blocks, I would
> prefer if you checked (addr + size) -- and not just addr -- against
> block["target_end"].

done

> 
> > +                haddr = block["host_addr"] + (addr -
> > block["target_start"])
> > +                return qemu_core.read_memory(haddr, size)
> > +
> > +    def add_vmcoreinfo(self):
> > +        if not gdb.parse_and_eval("vmcoreinfo_gdb_helper"):
> > +            return
> > +
> > +        addr =
> > gdb.parse_and_eval("vmcoreinfo_gdb_helper.vmcoreinfo_addr_le")
> > +        addr = bytes([addr[i] for i in range(4)])
> > +        addr = struct.unpack("<I", addr)[0]
> > +
> > +        mem = self.phys_memory_read(addr, 16)
> > +        (version, addr, size) = struct.unpack("<IQI", mem)
> > +        if version != 0:
> > +            return
> > +
> > +        vmcoreinfo = self.phys_memory_read(addr, size)
> > +        if vmcoreinfo:
> > +            self.elf.add_vmcoreinfo_note(vmcoreinfo.tobytes())
> > +
> >      def invoke(self, args, from_tty):
> >          """Handles command invocation from gdb."""
> >  
> > @@ -518,6 +557,7 @@ shape and this command should mostly work."""
> >  
> >          self.elf = ELF(argv[1])
> >          self.guest_phys_blocks = get_guest_phys_blocks()
> > +        self.add_vmcoreinfo()
> >  
> >          with open(argv[0], "wb") as vmcore:
> >              self.dump_init(vmcore)
> > diff --git a/hw/acpi/vmcoreinfo.c b/hw/acpi/vmcoreinfo.c
> > index 0ea41de8d9..b6bcb47506 100644
> > --- a/hw/acpi/vmcoreinfo.c
> > +++ b/hw/acpi/vmcoreinfo.c
> > @@ -163,6 +163,8 @@ static void vmcoreinfo_handle_reset(void *opaque)
> >      memset(vis->vmcoreinfo_addr_le, 0,
> >      ARRAY_SIZE(vis->vmcoreinfo_addr_le));
> >  }
> >  
> > +static VMCoreInfoState *vmcoreinfo_gdb_helper;
> > +
> >  static void vmcoreinfo_realize(DeviceState *dev, Error **errp)
> >  {
> >      if (!bios_linker_loader_can_write_pointer()) {
> > @@ -181,6 +183,7 @@ static void vmcoreinfo_realize(DeviceState *dev, Error
> > **errp)
> >          return;
> >      }
> >  
> > +    vmcoreinfo_gdb_helper = VMCOREINFO(dev);
> >      qemu_register_reset(vmcoreinfo_handle_reset, dev);
> >  }
> >  
> > 
> 
> I guess we don't build QEMU with link-time optimization at the moment.
> 
> With link-time optimization, I think gcc might reasonably optimize away
> the assignment to "vmcoreinfo_gdb_helper", and "vmcoreinfo_gdb_helper"
> itself. This is why I suggested "volatile":
> 
> static VMCoreInfoState * volatile vmcoreinfo_gdb_helper;
> 
> Do you think volatile is only superfluous, or do you actively dislike it
> for some reason?

Yeah, I am not convinced volatile is the best way, but nor is static.

Let's export it?