Re: [PATCH][RFC] parisc: Use local tlb purges only on UP machines

[Helge Deller <deller@gmx.de>]

On 9/25/22 21:27, John David Anglin wrote:
> On 2022-09-25 2:58 p.m., Helge Deller wrote:
>> On 9/25/22 20:44, John David Anglin wrote:
>>> On 2022-09-25 2:19 p.m., John David Anglin wrote:
>>>>> One goal of that patch was to drop the CONFIG_PA20 ifdef case,
>>>>> because a 32-bit kernel could be compiled for PA8000 in which case
>>>>> the "pdtlb,l" will burn the machine.
>>>> Don't think so.  "pdtlb,l" is available on all PA 2.0 machines. It's not 64-bit specific.
>>> There is some difference in implementation between PA 1.1 and 2.0. 64-bit register
>>> values are used in the PA 2.0 implementation.
>>
>> That's right.
>> But if you build a 32-bit kernel and choose to optimize for PA8x00 CPUs,
>> the CONFIG_PA20 is set and the local-purge is used unconditionally.
>> That breaks e.g. when running such a kernel in qemu (which is 32-bit only).
> I don't think that's a valid kernel configuration for qemu.

I agree - it's probably not the best choice if you want to run qemu...

> It only supports the PA 1.1 instruction set.  PA8x00 CPUs always
> support the PA 2.0 instruction set even when running in 32-bit mode.
The 32-bit kernel is built with the 32-bit compiler.
As far as I understand arch/parisc/Makefile, choosing the CPU type
just enables *tuning* for the selected CPU. See Makefile:

# select which processor to optimise for
cflags-$(CONFIG_PA7000)         += -march=1.1 -mschedule=7100
cflags-$(CONFIG_PA7200)         += -march=1.1 -mschedule=7200
cflags-$(CONFIG_PA7100LC)       += -march=1.1 -mschedule=7100LC
cflags-$(CONFIG_PA7300LC)       += -march=1.1 -mschedule=7300
cflags-$(CONFIG_PA8X00)         += -march=2.0 -mschedule=8000

The only assembler instructions which break qemu are our manually
added ones: "pdtlb,l" and "ldd" (for flushing - see my first patch).
The two patches I sent today allows to boot such a kernel, and
they still keep the PA2.0 support when run on PA2.0 machines.

Helge