[PATCH v2 2/3] powerpc/64s: remove PROT_SAO support

[Nicholas Piggin]

Excerpts from Paul Mackerras's message of July 9, 2020 2:34 pm:
> On Fri, Jul 03, 2020 at 11:19:57AM +1000, Nicholas Piggin wrote:
>> ISA v3.1 does not support the SAO storage control attribute required to
>> implement PROT_SAO. PROT_SAO was used by specialised system software
>> (Lx86) that has been discontinued for about 7 years, and is not thought
>> to be used elsewhere, so removal should not cause problems.
>> 
>> We rather remove it than keep support for older processors, because
>> live migrating guest partitions to newer processors may not be possible
>> if SAO is in use (or worse allowed with silent races).
> 
> This is actually a real problem for KVM, because now we have the
> capabilities of the host affecting the characteristics of the guest
> virtual machine in a manner which userspace (e.g. QEMU) is unable to
> control.
> 
> It would probably be better to disallow SAO on all machines than have
> it available on some hosts and not others.  (Yes I know there is a
> check on CPU_FTR_ARCH_206 in there, but that has been a no-op since we
> removed the PPC970 KVM support.)

This change doesn't change the SAO difference on the host processors
though, just tries to slightly improve it from silently broken to
maybe complaining a bit.

I didn't want to stop some very old image that uses this and is running
okay on an existing host from working, but maybe the existence of such
a thing would contradict my reasoning. But then if we don't care about
it why care about this KVM behaviour difference at all?

> Solving this properly will probably require creating a new KVM host
> capability and associated machine parameter in QEMU, along with a new
> machine type.

Rather than answer any of these questions, I might take the KVM change
out and that can be dealt with separately from guest SAO removal.

Thanks,
Nick

> 
> [snip]
> 
>> diff --git a/arch/powerpc/include/asm/kvm_book3s_64.h b/arch/powerpc/include/asm/kvm_book3s_64.h
>> index 9bb9bb370b53..fac39ff659d4 100644
>> --- a/arch/powerpc/include/asm/kvm_book3s_64.h
>> +++ b/arch/powerpc/include/asm/kvm_book3s_64.h
>> @@ -398,9 +398,10 @@ static inline bool hpte_cache_flags_ok(unsigned long hptel, bool is_ci)
>>  {
>>  	unsigned int wimg = hptel & HPTE_R_WIMG;
>>  
>> -	/* Handle SAO */
>> +	/* Handle SAO for POWER7,8,9 */
>>  	if (wimg == (HPTE_R_W | HPTE_R_I | HPTE_R_M) &&
>> -	    cpu_has_feature(CPU_FTR_ARCH_206))
>> +	    cpu_has_feature(CPU_FTR_ARCH_206) &&
>> +	    !cpu_has_feature(CPU_FTR_ARCH_31))
>>  		wimg = HPTE_R_M;
> 
> Paul.
>