Re: [PATCH][RFC] parisc: Use local tlb purges only on PA2.0 machines

[Helge Deller <deller@gmx.de>]

On 9/25/22 22:28, John David Anglin wrote:
> On 2022-09-25 4:11 p.m., Helge Deller wrote:
>> n 9/25/22 22:02, John David Anglin wrote:
>>> On 2022-09-25 2:57 p.m., Helge Deller wrote:
>>>> +#ifdef CONFIG_PA20
>>>> +#define ALT_COND_PACACHE    ALT_COND_ALWAYS
>>>> +#else
>>>> +#define ALT_COND_PACACHE    ALT_COND_NO_SMP
>>>> +#endif
>>>> +
>>>>   ENTRY_CFI(flush_tlb_all_local)
>>>>       /*
>>>>        * The pitlbe and pdtlbe instructions should only be used to
>>>> @@ -539,15 +545,10 @@ ENTRY_CFI(copy_user_page_asm)
>>>>
>>>>       /* Purge any old translations */
>>>>
>>>> -#ifdef CONFIG_PA20
>>>> -    pdtlb,l        %r0(%r28)
>>>> -    pdtlb,l        %r0(%r29)
>>>> -#else
>>>>   0:    pdtlb        %r0(%r28)
>>>>   1:    pdtlb        %r0(%r29)
>>>> -    ALTERNATIVE(0b, 0b+4, ALT_COND_NO_SMP, INSN_PxTLB)
>>>> -    ALTERNATIVE(1b, 1b+4, ALT_COND_NO_SMP, INSN_PxTLB)
>>>> -#endif
>>>> +    ALTERNATIVE(0b, 0b+4, ALT_COND_PACACHE, INSN_PxTLB)
>>>> +    ALTERNATIVE(1b, 1b+4, ALT_COND_PACACHE, INSN_PxTLB)
>>> This doesn't look correct.  If ALT_COND_PACACHE is defined as ALT_COND_NO_SMP, the pdtlb
>>> instructions will be converted to pdtlb,l instructions when running UP.  These are not supported
>>> on PA 1.1.
>>
>> Your concern is correct, but there is an additonal check in the alternative-coding,
>> which prevents enabling the local flag if we're not running on a PA2.0 CPU.
>> So, those ALTERNATIVE() macros will only apply on PA2.0 machines.
> You are correct.  Missed that.
>
> That only leaves the bus serialization issue when pdtlb is used on an SMP machine.

I think we are Ok with what's in the kernel already.

According to arch/parisc/include/asm/pgtable.h:

* This is for the serialization of PxTLB broadcasts. At least on the N class
  * systems, only one PxTLB inter processor broadcast can be active at any one
  * time on the Merced bus. */
extern spinlock_t pa_tlb_flush_lock;
#if defined(CONFIG_64BIT) && defined(CONFIG_SMP)
extern int pa_serialize_tlb_flushes;
#else
#define pa_serialize_tlb_flushes        (0)
#endif

we currently do TLB serialization on 64-bit machines with a 64-bit kernel only.
N-class machines are 64-bit-only machines which can't run a 32-bit kernel.
So, 32-bit SMP kernels (which don't have serialization for PxTLB flushes)
don't seem to be affected.

Helge