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

[Eduardo Habkost]

On Wed, Jan 10, 2018 at 01:30:29PM -0800, Alistair Francis wrote:
> On Thu, Dec 28, 2017 at 6:59 AM, Eduardo Habkost <address@hidden> wrote:
> > On Thu, Dec 28, 2017 at 02:39:31PM +0100, Igor Mammedov wrote:
> >> On Fri, 22 Dec 2017 11:47:00 -0800
> >> Alistair Francis <address@hidden> wrote:
> >>
> >> > On Fri, Dec 22, 2017 at 10:45 AM, Alistair Francis
> >> > <address@hidden> wrote:
> >> > > On Wed, Dec 20, 2017 at 2:06 PM, Eduardo Habkost <address@hidden> 
> >> > > 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
> >> > >>> <address@hidden> wrote:
> >> > >>> > On Tue, Dec 19, 2017 at 4:43 PM, Peter Maydell <address@hidden> 
> >> > >>> > wrote:
> >> > >>> >> On 20 December 2017 at 00:27, Alistair Francis
> >> > >>> >> <address@hidden> 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?
> >> It's sort of going backwards but I won't object to this as far as you
> >> won't use machine->cpu_model (which is in process of being removed)
> 
> Wait, machine->cpu_model is the human readable name. Without using
> that we can't use just human readable strings for the valid cpu types.

Well, if we want to deprecate machine->cpu_model we need to offer
an alternative first, otherwise we can't prevent people from
using it.

Igor, do you see an (existing) alternative to machine->cpu_model
that would allow us to avoid using it in
machine_run_board_init()?

-- 
Eduardo