Re: [Qemu-devel] [PATCH v4 0/5] Add a valid_cpu_types property

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Dec 22, 2017 at 11:47:00AM -0800, Alistair Francis wrote:
> On Fri, Dec 22, 2017 at 10:45 AM, Alistair Francis
> <alistair.francis@xilinx.com> wrote:
> > On Wed, Dec 20, 2017 at 2:06 PM, Eduardo Habkost <ehabkost@redhat.com> wrote:
> >> On Tue, Dec 19, 2017 at 05:03:59PM -0800, Alistair Francis wrote:
> >>> On Tue, Dec 19, 2017 at 4:55 PM, Alistair Francis
> >>> <alistair.francis@xilinx.com> wrote:
> >>> > On Tue, Dec 19, 2017 at 4:43 PM, Peter Maydell <peter.maydell@linaro.org> wrote:
> >>> >> On 20 December 2017 at 00:27, Alistair Francis
> >>> >> <alistair.francis@xilinx.com> wrote:
> >>> >>> There are numorous QEMU machines that only have a single or a handful of
> >>> >>> valid CPU options. To simplyfy the management of specificying which CPU
> >>> >>> is/isn't valid let's create a property that can be set in the machine
> >>> >>> init. We can then check to see if the user supplied CPU is in that list
> >>> >>> or not.
> >>> >>>
> >>> >>> I have added the valid_cpu_types for some ARM machines only at the
> >>> >>> moment.
> >>> >>>
> >>> >>> Here is what specifying the CPUs looks like now:
> >>> >>>
> >>> >>> $ aarch64-softmmu/qemu-system-aarch64 -M netduino2 -kernel ./u-boot.elf -nographic -cpu "cortex-m3" -S
> >>> >>> QEMU 2.10.50 monitor - type 'help' for more information
> >>> >>> (qemu) info cpus
> >>> >>> * CPU #0: thread_id=24175
> >>> >>> (qemu) q
> >>> >>>
> >>> >>> $ aarch64-softmmu/qemu-system-aarch64 -M netduino2 -kernel ./u-boot.elf -nographic -cpu "cortex-m4" -S
> >>> >>> QEMU 2.10.50 monitor - type 'help' for more information
> >>> >>> (qemu) q
> >>> >>>
> >>> >>> $ aarch64-softmmu/qemu-system-aarch64 -M netduino2 -kernel ./u-boot.elf -nographic -cpu "cortex-m5" -S
> >>> >>> qemu-system-aarch64: unable to find CPU model 'cortex-m5'
> >>> >>>
> >>> >>> $ aarch64-softmmu/qemu-system-aarch64 -M netduino2 -kernel ./u-boot.elf -nographic -cpu "cortex-a9" -S
> >>> >>> qemu-system-aarch64: Invalid CPU type: cortex-a9-arm-cpu
> >>> >>> The valid types are: cortex-m3-arm-cpu, cortex-m4-arm-cpu
> >>> >>
> >>> >> Thanks for this; we really should be more strict about
> >>> >> forbidding "won't work" combinations than we have
> >>> >> been in the past.
> >>> >>
> >>> >> In the last of these cases, I think that when we
> >>> >> list the invalid CPU type and the valid types
> >>> >> we should use the same names we want the user to
> >>> >> use on the command line, without the "-arm-cpu"
> >>> >> suffixes.
> >>> >
> >>> > Hmm... That is a good point, it is confusing that they don't line up.
> >>
> >> Agreed.
> >>
> >>> >
> >>> > The problem is that we are just doing a simple
> >>> > object_class_dynamic_cast() in hw/core/machine.c which I think
> >>> > (untested) requires us to have the full name in the valid cpu array.
> >> [...]
> >>>
> >>> I think an earlier version of my previous series adding the support to
> >>> machine.c did string comparison, but it was decided to utilise objects
> >>> instead. One option is to make the array 2 wide and have the second
> >>> string be user friendly?
> >>
> >> Making the array 2-column will duplicate information that we can
> >> already find out using other methods, and it won't solve the
> >> problem if an entry has a parent class with multiple subclasses
> >> (the original reason I suggested object_class_dynamic_cast()).
> >>
> >> The main obstacle to fix this easily is that we do have a common
> >>   ObjectClass *cpu_class_by_name(const char *cpu_model)
> >> function, but not a common method to get the model name from a
> >> CPUClass.  Implementing this is possible, but probably better to
> >> do it after moving the existing arch-specific CPU model
> >> enumeration hooks to common code (currently we duplicate lots of
> >> CPU enumeration/lookup boilerplate code that we shouldn't have
> >> to).
> >>
> >> Listing only the human-friendly names in the array like in the
> >> original patch could be a reasonable temporary solution.  It
> >> won't allow us to use a single entry for all subclasses of a
> >> given type by now (e.g. listing only TYPE_X86_CPU on PC), but at
> >> least we can address this issue without waiting for a refactor of
> >> the CPU model enumeration code.
> 
> Ah, I just re-read this. Do you mean go back to the original RFC and
> just use strcmp() to compare the human readable cpu_model?

Yes, I think this would be simpler, considering that the current
users don't need the object_class_dynamic_cast() stuff.

Then whoever decide to make PC simply list TYPE_X86_CPU later
(possibly me) will need to clean up the CPU model
enumeration/lookup code and implement a
cpu_model_from_type_name() helper.

-- 
Eduardo