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

[Paul Mackerras]

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.)

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

[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.