On 24/11/2024 13.28, Philippe Mathieu-Daudé wrote:
On 24/11/24 01:21, Pierrick Bouvier wrote:
Hi all,
After thinking about it, a simple, exhaustive and reliable way to find
this type information is the debug (dwarf) info.
By compiling qemu binaries with --enable-debug, and extracting info using
llvm-dwarfdump plus a custom filter [4], we can obtain a text
representation of all structures QEMU uses.
As expected, turning the option globally has an effect. The exact list of
structures impacted (those having bitfields, and not being packed) can be
seen for each binary here: [5].
As there is a lot of repetition between all qemu binaries, the reduced
list of structs concerned is [6]:
+name:ArduinoMachineClass size:0x0198
+name:ARMCacheAttrs size:0x04
+name:ARMVAParameters size:0x04
+name:AspeedMachineClass size:0x01d0
+name:_GIOChannel size:0x70
+name:MachineClass size:0x0188
+name:MicrovmMachineClass size:0x01a0
+name:MPS2MachineClass size:0x01a8
+name:MPS2TZMachineClass size:0x01e8
+name:MPS3RMachineClass size:0x01a0
+name:MuscaMachineClass size:0x01a8
+name:NPCM7xxMachineClass size:0x0190
+name:PCMachineClass size:0x01c0
+name:PnvMachineClass size:0x01b0
+name:PPCE500MachineClass size:0x01e0
+name:RaspiBaseMachineClass size:0x0190
+name:RxGdbSimMachineClass size:0x0198
+name:S390CcwMachineClass size:0x0190
+name:SpaprMachineClass size:0x01d0
+name:Sun4mMachineClass size:0x0190
+name:TriBoardMachineClass size:0x01a0
+name:VexpressMachineClass size:0x0190
+name:VirtMachineClass size:0x01a0
+name:X86MachineClass size:0x0190
The *MachineClass ones come from [*]:
struct MachineClass {
...
unsigned int no_serial:1,
no_parallel:1,
no_floppy:1,
no_cdrom:1,
no_sdcard:1,
pci_allow_0_address:1,
legacy_fw_cfg_order:1;
IMHO Using bitfield to manually micro-optimize memory size seems
a bit a waste of developer time/focus, I'd rather see compilers
doing that for us.
Regards,
Phil.
[*] Interestingly Thomas is also looking at those fields:
https://lore.kernel.org/qemu-devel/20241122084923.1542743-1-th...@redhat.com/
Yes, I got curious by the msbitfield discussion last week and remembered
that we had a look at the differences of the structs with "pahole" in the
past already, so I wondered what might be the difference nowadays, and the
machine class is the most prominent difference, indeed, that's why I came up
with that patch.
Thomas
