[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH for-4.1] q35: acpi: do not create dummy MCFG tab
From: |
Igor Mammedov |
Subject: |
Re: [Qemu-devel] [PATCH for-4.1] q35: acpi: do not create dummy MCFG table |
Date: |
Thu, 11 Apr 2019 13:46:27 +0200 |
On Thu, 11 Apr 2019 09:32:11 +0800
Wei Yang <address@hidden> wrote:
> On Wed, Apr 10, 2019 at 05:01:50PM +0200, Igor Mammedov wrote:
> >On Wed, 10 Apr 2019 22:27:56 +0800
> >Wei Yang <address@hidden> 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.
well, it works fine for me.
Where do you see max_length being used during migration?
> 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?
yes, you did, max_length does not influence migration stream.
see what above mentioned commits and ram_load() -> "if (length !=
block->used_length)" do.
Re: [Qemu-devel] [PATCH for-4.1] q35: acpi: do not create dummy MCFG table, Wei Yang, 2019/04/11