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

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 3 Apr 2019 16:25:49 +0000
Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com> wrote:

> Hi Laszlo,
> 
> > -----Original Message-----
> > From: Laszlo Ersek [mailto:lersek@redhat.com]
> > Sent: 03 April 2019 14:29
> > To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>;
> > Igor Mammedov <imammedo@redhat.com>
> > Cc: Auger Eric <eric.auger@redhat.com>; Ard Biesheuvel
> > <ard.biesheuvel@linaro.org>; peter.maydell@linaro.org;
> > sameo@linux.intel.com; qemu-devel@nongnu.org; Linuxarm
> > <linuxarm@huawei.com>; shannon.zhaosl@gmail.com;
> > qemu-arm@nongnu.org; xuwei (O) <xuwei5@huawei.com>;
> > sebastien.boeuf@intel.com; Leif Lindholm <Leif.Lindholm@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.
Lets try adding a parameter to memory descriptors in DT that would mark
them as hotpluggable.

 
> Cheers,
> Shameer