Re: [PATCH] KVM: PPC: Book3S HV: Check for updated HDSISR on P9 HDSI exception

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Sep 15, 2017 at 03:26:14PM +1000, Michael Neuling wrote:
> On POWER9 DD2.1 and below, sometimes on a Hypervisor Data Storage
> Interrupt (HDSI) the HDSISR is not be updated at all.
> 
> To work around this we put a canary value into the HDSISR before
> returning to a guest and then check for this canary when we take a
> HDSI. If we find the canary on a HDSI, we know the hardware didn't
> update the HDSISR. In this case we return to the guest to retake the
> HDSI which should correctly update the HDSISR the second time HDSI
> entry.
> 
> After talking to Paulus we've applied this workaround to all POWER9
> CPUs. The workaround of returning to the guest shouldn't ever be
> triggered on well behaving CPU. The extra instructions should have
> negligible performance impact.
> 
> Signed-off-by: Michael Neuling <mikey@neuling.org>

Paolo,

Any chance you can take this directly into your staging tree.  This is
a workaround for a potentially serious hardware bug, and we want to
get it into the downstream release ASAP.  However, Paulus is away for
a couple of weeks, so we can't count on him staging it in the kvm-ppc
tree soon.

> ---
>  arch/powerpc/kvm/book3s_hv_rmhandlers.S | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> index 663a4a861e..70dca60569 100644
> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> @@ -1118,6 +1118,13 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  BEGIN_FTR_SECTION
>  	mtspr	SPRN_PPR, r0
>  END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
> +
> +/* Move canary into DSISR to check for later */
> +BEGIN_FTR_SECTION
> +	li	r0, 0x7fff
> +	mtspr	SPRN_HDSISR, r0
> +END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
> +
>  	ld	r0, VCPU_GPR(R0)(r4)
>  	ld	r4, VCPU_GPR(R4)(r4)
>  
> @@ -1947,9 +1954,14 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_RADIX)
>  kvmppc_hdsi:
>  	ld	r3, VCPU_KVM(r9)
>  	lbz	r0, KVM_RADIX(r3)
> -	cmpwi	r0, 0
>  	mfspr	r4, SPRN_HDAR
>  	mfspr	r6, SPRN_HDSISR
> +BEGIN_FTR_SECTION
> +	/* Look for DSISR canary. If we find it, retry instruction */
> +	cmpdi	r6, 0x7fff
> +	beq	6f
> +END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
> +	cmpwi	r0, 0
>  	bne	.Lradix_hdsi		/* on radix, just save DAR/DSISR/ASDR */
>  	/* HPTE not found fault or protection fault? */
>  	andis.	r0, r6, (DSISR_NOHPTE | DSISR_PROTFAULT)@h

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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