Hi Laszlo, > -----Original Message----- > From: Laszlo Ersek [mailto:ler...@redhat.com] > Sent: 03 April 2019 14:29 > To: Shameerali Kolothum Thodi <shameerali.kolothum.th...@huawei.com>; > Igor Mammedov <imamm...@redhat.com> > Cc: Auger Eric <eric.au...@redhat.com>; Ard Biesheuvel > <ard.biesheu...@linaro.org>; peter.mayd...@linaro.org; > sa...@linux.intel.com; qemu-devel@nongnu.org; Linuxarm > <linux...@huawei.com>; shannon.zha...@gmail.com; > qemu-...@nongnu.org; xuwei (O) <xuw...@huawei.com>; > sebastien.bo...@intel.com; Leif Lindholm <leif.lindh...@arm.com> > Subject: Re: [Qemu-devel] [PATCH v3 07/10] hw/arm/virt: Introduce opt-in > feature "fdt" > > On 04/03/19 14:10, Shameerali Kolothum Thodi wrote: > > >>>>> So, the condition for hiding the hotpluggable memory nodes in question > >>>>> from the DT is: > >>>> > >>>>> > >>>>> (aarch64 && firmware_loaded && acpi_enabled) > > >>> While UEFI has bindings for both 32-bit and 64-bit ARM, ACPI has a > >>> 64-bit-only binding for ARM. (And you can have UEFI without ACPI, but > >>> not the reverse, on ARM.) So if you run the 32-bit build of the > >>> ArmVirtQemu firmware, you get no ACPI at all; all you can rely on with > >>> the OS is the DT. > > > > Just to confirm, does that mean with 32-bit build of the UEFI, the OS cannot > > boot with ACPI and uses DT only. > > Indeed. > > > So, > > > > If ((aarch64 && firmware_loaded && acpi_enabled) { > > Hide_hotpluggable_memory_nodes() > > } else { > > Add_ hotpluggable_memory_nodes() > > } > > > > should work for all cases? > > Yes. > > Here's what happens when any one of the subconditions evaluates to false: > > - ARM32 has no ACPI bindings, so the guest kernel can only use DT. > > - On AARCH64, if you don't "load the firmware" (= don't use UEFI), then > there won't be an ACPI entry point for the OS to locate (the RSD PTR > is defined by the ACPI spec in UEFI terms, for AARCH64). So the guest > kernel can only use DT. > > - When on AARCH64 and using UEFI, but asking QEMU not to generate ACPI > content, the firmware will not install any ACPI tables, so the guest > kernel can only use DT. >
Thanks. That makes it very clear. Much appreciated. > >>> This "bitness distinction" is implemented in the firmware already. If > >>> you hid the memory nodes from the DT under the condition > >>> > >>> (!aarch64 && firmware_loaded && acpi_enabled) > >>> > >>> then the nodes would not be seen by the OS at all (because > >>> "acpi_enabled" is irrelevant for the 32-bit build of ArmVirtQemu, and > >>> all the OS can ever get is DT). > >> > >> It's getting tricky and I don't like a bit that we are trying to carter > >> 64 bit only UEFI build (or any other build) on QEMU side. Also Peter has > >> a valid about guessing on QEMU side (that's usually a source of problem > >> in the future). > > > > If the above is correct(with 32-bit variant of UEFI, OS cannot have ACPI > > boot), > > then do we really have the issue of memory becoming non > hot-un-unpluggable? > > May be I am missing something. > > I think Igor and Peter dislike adding complex logic to QEMU that > reflects the behavior of a specific firmware. AIUI their objection isn't > that it wouldn't work, but that it's not the right thing to do, from a > design perspective. Understood. Hope we can converge on something soon. Cheers, Shameer
