On 05/08/19 12:15, Shameerali Kolothum Thodi wrote: > Hi, > > This series here[0] attempts to add support for PCDIMM in QEMU for > ARM/Virt platform and has stumbled upon an issue as it is not clear(at least > from Qemu/EDK2 point of view) how in physical world the hotpluggable > memory is handled by kernel. > > The proposed implementation in Qemu, builds the SRAT and DSDT parts > and uses GED device to trigger the hotplug. This works fine. > > But when we added the DT node corresponding to the PCDIMM(cold plug > scenario), we noticed that Guest kernel see this memory during early boot > even if we are booting with ACPI. Because of this, hotpluggable memory > may end up in zone normal and make it non-hot-un-pluggable even if Guest > boots with ACPI. > > Further discussions[1] revealed that, EDK2 UEFI has no means to interpret the > ACPI content from Qemu(this is designed to do so) and uses DT info to > build the GetMemoryMap(). To solve this, introduced "hotpluggable" property > to DT memory node(patches #7 & #8 from [0]) so that UEFI can differentiate > the nodes and exclude the hotpluggable ones from GetMemoryMap(). > > But then Laszlo rightly pointed out that in order to accommodate the changes > into UEFI we need to know how exactly Linux expects/handles all the > hotpluggable memory scenarios. Please find the discussion here[2]. > > For ease, I am just copying the relevant comment from Laszlo below, > > /****** > "Given patches #7 and #8, as I understand them, the firmware cannot distinguish > hotpluggable & present, from hotpluggable & absent. The firmware can only > skip both hotpluggable cases. That's fine in that the firmware will hog neither > type -- but is that OK for the OS as well, for both ACPI boot and DT boot? > > Consider in particular the "hotpluggable & present, ACPI boot" case. Assuming > we modify the firmware to skip "hotpluggable" altogether, the UEFI memmap > will not include the range despite it being present at boot. Presumably, ACPI > will refer to the range somehow, however. Will that not confuse the OS? > > When Igor raised this earlier, I suggested that hotpluggable-and-present should > be added by the firmware, but also allocated immediately, as EfiBootServicesData > type memory. This will prevent other drivers in the firmware from allocating AcpiNVS > or Reserved chunks from the same memory range, the UEFI memmap will contain > the range as EfiBootServicesData, and then the OS can release that allocation in > one go early during boot. > > But this really has to be clarified from the Linux kernel's expectations. Please > formalize all of the following cases: > > OS boot (DT/ACPI) hotpluggable & ... GetMemoryMap() should report as DT/ACPI should report as > ----------------- ------------------ ------------------------------- ------------------------ > DT present ? ? > DT absent ? ? > ACPI present ? ? > ACPI absent ? ? > > Again, this table is dictated by Linux." > > ******/ > > Could you please take a look at this and let us know what is expected here from > a Linux kernel view point. > > (Hi Laszlo/Igor/Eric, please feel free to add/change if I have missed any valid > points above). I'm happy with your summary, thank you! Laszlo > > Thanks, > Shameer > [0] https://patchwork.kernel.org/cover/10890919/ > [1] https://patchwork.kernel.org/patch/10863299/ > [2] https://patchwork.kernel.org/patch/10890937/ > >
