Re: [PATCH v1] KVM: PPC: Book3S HV: Prevent POWER7/8 TLB flush flushing SLB

[Fabiano Rosas <farosas@linux.ibm.com>]

Nicholas Piggin <npiggin@gmail.com> writes:

> The POWER9 ERAT flush instruction is a SLBIA with IH=7, which is a
> reserved value on POWER7/8. On POWER8 this invalidates the SLB entries
> above index 0, similarly to SLBIA IH=0.
>
> If the SLB entries are invalidated, and then the guest is bypassed, the
> host SLB does not get re-loaded, so the bolted entries above 0 will be
> lost. This can result in kernel stack access causing a SLB fault.
>
> Kernel stack access causing a SLB fault was responsible for the infamous
> mega bug (search "Fix SLB reload bug"). Although since commit
> 48e7b7695745 ("powerpc/64s/hash: Convert SLB miss handlers to C") that
> starts using the kernel stack in the SLB miss handler, it might only
> result in an infinite loop of SLB faults. In any case it's a bug.
>
> Fix this by only executing the instruction on >= POWER9 where IH=7 is
> defined not to invalidate the SLB. POWER7/8 don't require this ERAT
> flush.
>
> Fixes: 5008711259201 ("KVM: PPC: Book3S HV: Invalidate ERAT when flushing guest TLB entries")
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>

> ---
>  arch/powerpc/kvm/book3s_hv_builtin.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv_builtin.c b/arch/powerpc/kvm/book3s_hv_builtin.c
> index fcf4760a3a0e..70b7a8f97153 100644
> --- a/arch/powerpc/kvm/book3s_hv_builtin.c
> +++ b/arch/powerpc/kvm/book3s_hv_builtin.c
> @@ -695,6 +695,7 @@ static void flush_guest_tlb(struct kvm *kvm)
>  				       "r" (0) : "memory");
>  		}
>  		asm volatile("ptesync": : :"memory");
> +		// POWER9 congruence-class TLBIEL leaves ERAT. Flush it now.
>  		asm volatile(PPC_RADIX_INVALIDATE_ERAT_GUEST : : :"memory");
>  	} else {
>  		for (set = 0; set < kvm->arch.tlb_sets; ++set) {
> @@ -705,7 +706,9 @@ static void flush_guest_tlb(struct kvm *kvm)
>  			rb += PPC_BIT(51);	/* increment set number */
>  		}
>  		asm volatile("ptesync": : :"memory");
> -		asm volatile(PPC_ISA_3_0_INVALIDATE_ERAT : : :"memory");
> +		// POWER9 congruence-class TLBIEL leaves ERAT. Flush it now.
> +		if (cpu_has_feature(CPU_FTR_ARCH_300))
> +			asm volatile(PPC_ISA_3_0_INVALIDATE_ERAT : : :"memory");
>  	}
>  }