On Wed, Apr 10, 2019 at 05:01:50PM +0200, Igor Mammedov wrote: >On Wed, 10 Apr 2019 22:27:56 +0800 >Wei Yang <richardw.y...@linux.intel.com> wrote: > >[...] >> >@@ -2411,19 +2410,7 @@ build_mcfg_q35(GArray *table_data, BIOSLinker >> >*linker, AcpiMcfgInfo *info) >> > mcfg->allocation[0].start_bus_number = 0; >> > mcfg->allocation[0].end_bus_number = PCIE_MMCFG_BUS(info->mcfg_size - >> > 1); >> > >> >- /* MCFG is used for ECAM which can be enabled or disabled by guest. >> >> I want to cnfirm what is "enabled or disabled by guest" here. > >Firmware theoretically during PCI initialization may disable ECAM support >and that's when we do no need MCFG. In practice that's not happening >(SeaBIOS or UEFI) but we in case there is out there a firmware that does >disable ECAM we do not generate MCFG. > >Note: >ACPI tables generated twice, 1st when QEMU starts and the second time >when firmware accesses fwcfg to read blobs for the 1st time. >The later happens after PCI subsystem was initialized by firmware. >At that time we know if ECAM was enabled or not. >
That's much clear, thanks :-) So this is the guest BIOS instead of guest kernel who may disable/enable it. >> If we don't reserve mcfg and "guest" enable mcfg during running, the ACPI >> table size changed. But the destination still has the original table size, >> since destination "guest" keep sleep during this period. >> >> Now the migration would face table size difference > >with commit a1666142db we do not care as all the tables created on >source will be migrated to destination as is overwriting whatever blobs >destination created on startup. > >> and break migration? >nope, > >to help you figure out why it works >look at what following git commits did: > git log c8d6f66ae7..a1666142db >and pay attention to 'used_length' > To be honest, this is what I feel confused in your previous reply. First I want to confirm both fields in RAMBlock affects the migration: * used_length * max_length Both of them should be the same on both source/destination, otherwise the migration would fail. Then I thought the migration would be broken if source/destination has different knowledge about acpi table size. Because this will introduce different value of used_length, even we have resizable MemoryRegion. The 1st time ACPI generation flow: acpi_add_rom_blob rom_add_blob rom_set_mr memory_region_init_resizable_ram qemu_ram_alloc_resizable new_block->used_length = size new_block->max_length = max_size The 2nd time ACPI generation flow: acpi_ram_update memory_regioin_ram_resize qemu_ram_resize block->used_length = new_size The max_length is always the same, while used_length would be changed to the actual table_blob size. In case source/destination has different knowledge about acpi table size, the table_blob size(even after aligned) could be different. This is why I thought there is still some chance to break migration after resizable MemoryRegion. Do I miss something? -- Wei Yang Help you, Help me