Re: [Qemu-arm] [Qemu-devel] [PATCH v3 07/10] hw/arm/virt: Introduce opt-in feature "fdt"

[Auger Eric]

Hi Laszlo,

On 4/2/19 12:33 PM, Laszlo Ersek wrote:
> On 04/02/19 09:42, Igor Mammedov wrote:
>> On Mon, 1 Apr 2019 15:07:05 +0200
>> Laszlo Ersek <address@hidden> wrote:
>>
>>> On 03/29/19 14:56, Auger Eric wrote:
>>>> Hi Ard,
>>>>
>>>> On 3/29/19 2:14 PM, Ard Biesheuvel wrote:  
>>>>> On Fri, 29 Mar 2019 at 14:12, Auger Eric <address@hidden> wrote:  
>>>>>>
>>>>>> Hi Shameer,
>>>>>>
>>>>>> On 3/29/19 10:59 AM, Shameerali Kolothum Thodi wrote:  
>>>>>>>
>>>>>>>  
>>>>>>>> -----Original Message-----
>>>>>>>> From: Auger Eric [mailto:address@hidden
>>>>>>>> Sent: 29 March 2019 09:32
>>>>>>>> To: Shameerali Kolothum Thodi <address@hidden>;
>>>>>>>> address@hidden; address@hidden; address@hidden;
>>>>>>>> address@hidden; address@hidden;
>>>>>>>> address@hidden; address@hidden
>>>>>>>> Cc: Linuxarm <address@hidden>; xuwei (O) <address@hidden>;
>>>>>>>> Laszlo Ersek <address@hidden>; Ard Biesheuvel
>>>>>>>> <address@hidden>; Leif Lindholm <address@hidden>
>>>>>>>> Subject: Re: [PATCH v3 07/10] hw/arm/virt: Introduce opt-in feature 
>>>>>>>> "fdt"
>>>>>>>>
>>>>>>>> Hi Shameer,
>>>>>>>>
>>>>>>>> [ + Laszlo, Ard, Leif ]
>>>>>>>>
>>>>>>>> On 3/21/19 11:47 AM, Shameer Kolothum wrote:  
>>>>>>>>> This is to disable/enable populating DT nodes in case
>>>>>>>>> any conflict with acpi tables. The default is "off".  
>>>>>>>> The name of the option sounds misleading to me. Also we don't really
>>>>>>>> know the scope of the disablement. At the moment this just aims to
>>>>>>>> prevent the hotpluggable dt nodes from being added if we boot in ACPI 
>>>>>>>> mode.
>>>>>>>>  
>>>>>>>>>
>>>>>>>>> This will be used in subsequent patch where cold plug
>>>>>>>>> device-memory support is added for DT boot.  
>>>>>>>> I am concerned about the fact that in dt mode, by default, you won't 
>>>>>>>> see
>>>>>>>> any PCDIMM nodes.  
>>>>>>>>>
>>>>>>>>> If DT memory node support is added for cold-plugged device
>>>>>>>>> memory, those memory will be visible to Guest kernel via
>>>>>>>>> UEFI GetMemoryMap() and gets treated as early boot memory.  
>>>>>>>> Don't we have an issue in UEFI then. Normally the SRAT indicates 
>>>>>>>> whether
>>>>>>>> the slots are hotpluggable or not. Shouldn't the UEFI code look at this
>>>>>>>> info.  
>>>>>>>
>>>>>>> Sorry I missed this part. Yes, that will be a more cleaner solution.
>>>>>>>
>>>>>>> Also, to be more clear on what happens,
>>>>>>>
>>>>>>> Guest ACPI boot with "fdt=on" ,
>>>>>>>
>>>>>>> From kernel log,
>>>>>>>
>>>>>>> [    0.000000] Early memory node ranges
>>>>>>> [    0.000000]   node   0: [mem 0x0000000040000000-0x00000000bbf5ffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bbf60000-0x00000000bbffffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bc000000-0x00000000bc02ffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bc030000-0x00000000bc36ffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bc370000-0x00000000bf64ffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bf650000-0x00000000bf6dffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bf6e0000-0x00000000bf6effff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bf6f0000-0x00000000bf80ffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bf810000-0x00000000bfffffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000c0000000-0x00000000ffffffff]  
>>>>>>> --> This is the hotpluggable memory node from DT.
>>>>>>> [    0.000000] Zeroed struct page in unavailable ranges: 1040 pages
>>>>>>> [    0.000000] Initmem setup node 0 [mem 
>>>>>>> 0x0000000040000000-0x00000000ffffffff]
>>>>>>>
>>>>>>>
>>>>>>> Guest ACPI boot with "fdt=off" ,
>>>>>>>
>>>>>>> [    0.000000] Movable zone start for each node
>>>>>>> [    0.000000] Early memory node ranges
>>>>>>> [    0.000000]   node   0: [mem 0x0000000040000000-0x00000000bbf5ffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bbf60000-0x00000000bbffffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bc000000-0x00000000bc02ffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bc030000-0x00000000bc36ffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bc370000-0x00000000bf64ffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bf650000-0x00000000bf6dffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bf6e0000-0x00000000bf6effff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bf6f0000-0x00000000bf80ffff]
>>>>>>> [    0.000000]   node   0: [mem 0x00000000bf810000-0x00000000bfffffff]
>>>>>>> [    0.000000] Zeroed struct page in unavailable ranges: 1040 pages
>>>>>>> [    0.000000] Initmem setup node 0 [mem 
>>>>>>> 0x0000000040000000-0x00000000bfffffff
>>>>>>>
>>>>>>> The hotpluggable memory node is absent from early memory nodes here.  
>>>>>>
>>>>>> OK thank you for the example illustrating the concern.  
>>>>>>>
>>>>>>> As you said, it could be possible to detect this node using SRAT in 
>>>>>>> UEFI.  
>>>>>>
>>>>>> Let's wait for EDK2 experts on this.
>>>>>>  
>>>>>
>>>>> Happy to chime in, but I need a bit more context here.
>>>>>
>>>>> What is the problem, how does this path try to solve it, and why is
>>>>> that a bad idea?
>>>>>  
>>>> Sure, sorry.
>>>>
>>>> This series:
>>>> - [PATCH v3 00/10] ARM virt: ACPI memory hotplug support,
>>>> https://patchwork.kernel.org/cover/10863301/
>>>>
>>>> aims to introduce PCDIMM support in qemu. In ACPI mode, it builds the
>>>> SRAT and DSDT parts and relies on GED to trigger the hotplug.
>>>>
>>>> We noticed that if we build the hotpluggable memory dt nodes on top of
>>>> the above ACPI tables, the DIMM slots are interpreted as not
>>>> hotpluggable memory slots (at least we think so).
>>>>
>>>> We think the EDK2 GetMemoryMap() uses the dt node info and ignores the
>>>> fact that those slots are exposed as hotpluggable in the SRAT for example.
>>>>
>>>> So in this series, we are forced to not generate the hotpluggable memory
>>>> dt nodes if we want the DIMM slots to be effectively recognized as
>>>> hotpluggable.
>>>>
>>>> Could you confirm we have a correct understanding of the EDK2 behaviour
>>>> and if so, would there be any solution for EDK2 to absorb both the DT
>>>> nodes and the relevant SRAT/DSDT tables and make the slots hotpluggable.
>>>>
>>>> At qemu level, detecting we are booting in ACPI mode and purposely
>>>> removing the above mentioned DT nodes does not look straightforward.  
>>>
>>> The firmware is not enlightened about the ACPI content that comes from
>>> QEMU / fw_cfg. That ACPI content is *blindly* processed by the firmware,
>>> as instructed through the ACPI linker/loader script, in order to install
>>> the ACPI content for the OS. No actual information is consumed by the
>>> firmware from the ACPI payload -- and that's a feature.
>>>
>>> The firmware does consume DT:
>>>
>>> - If you start QEMU *with* "-no-acpi", then the DT is both consumed by
>>> the firmware (for its own information needs), and passed on to the OS.
>>>
>>> - If you start QEMU *without* "-no-acpi" (the default), then the DT is
>>> consumed only by the firmware (for its own information needs), and the
>>> DT is hidden from the OS. The OS gets only the ACPI content
>>> (processed/prepared as described above).
I am confused by the above statement actually. In the above case what
does happen if you pass the acpi=off in the kernel boot parameters?

Thanks

Eric
>>>
>>>
>>> In the firmware, the "ArmVirtPkg/HighMemDxe" driver iterates over the
>>> base/size pairs in all the memory nodes in the DT. For each such base
>>> address that is currently tracked as "nonexistent" in the GCD memory
>>> space map, the driver currently adds the base/size range as "system
>>> memory". This in turn is reflected by the UEFI memmap that the OS gets
>>> to see as "conventional memory".
>>>
>>> If you need some memory ranges to show up as "special" in the UEFI
>>> memmap, then you need to distinguish them somehow from the "regular"
>>> memory areas, in the DT. And then extend "ArmVirtPkg/HighMemDxe" in the
>>> firmware, so that it act upon the discriminator that you set in the DT.
>>>
>>>
>>> Now... from a brief look at the Platform Init and UEFI specs, my
>>> impression is that the hotpluggable (but presently not plugged) DIMM
>>> ranges should simply be *absent* from the UEFI memmap; is that correct?
>>> (I didn't check the ACPI spec, maybe it specifies the expected behavior
>>> in full.) If my impression is correct, then two options (alternatives)
>>> exist:
>>>
>>> (1) Hide the affected memory nodes -- or at least the affected base/size
>>> pairs -- from the DT, in case you boot without "-no-acpi" but with an
>>> external firmware loaded. Then the firmware will not expose those ranges
>>> as "conventional memory" in the UEFI memmap. This approach requires no
>>> changes to edk2.
>>>
>>> This option is precisely what Eric described up-thread, at
>>> <http://mid.mail-archive.com/address@hidden>:
>>>
>>>> in machvirt_init, there is firmware_loaded that tells you whether you
>>>> have a FW image. If this one is not set, you can induce dt. But if
>>>> there is a FW it can be either DT or ACPI booted. You also have the
>>>> acpi_enabled knob.  
>>>
>>> (The "-no-acpi" cmdline option clears the "acpi_enabled" variable in
>>> "vl.c").
>>>
>>> So, the condition for hiding the hotpluggable memory nodes in question
>>> from the DT is:
>>>
>>>   (aarch64 && firmware_loaded && acpi_enabled)
>> I'd go with this one, though I have a question for firmware side.
>> Let's assume we would want in future to expose hotpluggable & present
>> memory via GetMemoryMap() (like bare-metal does) (guest OS theoretically
>> can avoid using it for Normal zone based on hint from SRAT table early
>> at boot), but what about firmware can it inspect SRAT table and not use
>> hotpluggable ranges for its own use (or at least do not canibalize
>> them permanently)?
> 
> This is actually two questions:
> 
> (a) Can the firmware inspect SRAT?
> 
> If the SRAT table structure isn't very complex, this is technically
> doable, but the wrong thing to do, IMO.
> 
> First, we've tried hard to avoid enlightening the firmware about the
> semantics of QEMU's ACPI tables.
> 
> Second, this would introduce an ordering constraint (or callbacks) in
> the firmware, between the driver that processes & installs the ACPI
> tables, and the driver that translates the memory nodes of the DT to the
> memory ranges known to UEFI and the OS.
> 
> If we need such hinting, then option (2) below (from earlier context)
> would be better:
> - If it's OK to use an arm/aarch64 specific solution, then new DT
> properties should work.
> - If it should be arch-independent, then a dedicated fw_cfg file would
> be better.
> 
> (b) Assuming we have the information from some source, can the firmware
> expose some memory ranges as "usable RAM" to the OS, while staying away
> from them for its own (firmware) purposes?
> 
> After consulting
> 
>   Table 25. Memory Type Usage before ExitBootServices()
>   Table 26. Memory Type Usage after ExitBootServices()
> 
> in UEFI-2.7, I would say that the firmware driver that installs these
> ranges to the memory (space) map should also allocate the ranges right
> after, as EfiBootServicesData. This will prevent other drivers /
> applications in the firmware from allocating chunks out of those areas,
> and the OS will be at liberty to release and repurpose the ranges after
> ExitBootServices().
> 
> Thanks,
> Laszlo
> 
>>> (2) Invent and set an "ignore me, firmware" property for the
>>> hotpluggable memory nodes in the DT, and update the firmware to honor
>>> that property.
>>>
>>> Thanks
>>> Laszlo
>>
>