Re: [PATCH] arm64/io: Don't use WZR in writel

[Robin Murphy <robin.murphy@arm.com>]

On 12/03/2019 12:36, Marc Gonzalez wrote:
> On 24/02/2019 04:53, Bjorn Andersson wrote:
> 
>> On Sat 23 Feb 10:37 PST 2019, Marc Zyngier wrote:
>>
>>> On Sat, 23 Feb 2019 18:12:54 +0000, Bjorn Andersson wrote:
>>>>
>>>> On Mon 11 Feb 06:59 PST 2019, Marc Zyngier wrote:
>>>>
>>>>> On 11/02/2019 14:29, AngeloGioacchino Del Regno wrote:
>>>>>
>>>>>> Also, just one more thing: yes this thing is going ARM64-wide and
>>>>>> - from my findings - it's targeting certain Qualcomm SoCs, but...
>>>>>> I'm not sure that only QC is affected by that, others may as well
>>>>>> have the same stupid bug.
>>>>>
>>>>> At the moment, only QC SoCs seem to be affected, probably because
>>>>> everyone else has debugged their hypervisor (or most likely doesn't
>>>>> bother with shipping one).
>>>>>
>>>>> In all honesty, we need some information from QC here: which SoCs are
>>>>> affected, what is the exact nature of the bug, can it be triggered from
>>>>> EL0. Randomly papering over symptoms is not something I really like
>>>>> doing, and is likely to generate problems on unaffected systems.
>>>>
>>>> The bug at hand is that the XZR is not deemed a valid source in the
>>>> virtualization of the SMMU registers. It was identified and fixed for
>>>> all platforms that are shipping kernels based on v4.9 or later.
>>>
>>> When you say "fixed": Do you mean fixed in the firmware? Or by adding
>>> a workaround in the shipped kernel?
>>
>> I mean that it's fixed in the firmware.
>>
>>> If the former, is this part of an official QC statement, with an
>>> associated erratum number?
>>
>> I don't know, will get back to you on this one.
>>
>>> Is this really limited to the SMMU accesses?
>>
>> Yes.
>>
>>>> As such Angelo's list of affected platforms covers the high-profile
>>>> ones. In particular MSM8996 and MSM8998 is getting pretty good support
>>>> upstream, if we can figure out a way around this issue.
>>>
>>> We'd need an exhaustive list of the affected SoCs, and work out if we
>>> can limit the hack to the SMMU driver (cc'ing Robin, who's the one
>>> who'd know about it).
>>
>> I will try to compose a list.
> 
> FWIW, I have just been bitten by this issue. I needed to enable an SMMU to
> filter PCIe EP accesses to system RAM (or something). I'm using an APQ8098
> MEDIABOX dev board. My system hangs in arm_smmu_device_reset() doing:
> 
> 	/* Invalidate the TLB, just in case */
> 	writel_relaxed(0, gr0_base + ARM_SMMU_GR0_TLBIALLH);
> 	writel_relaxed(0, gr0_base + ARM_SMMU_GR0_TLBIALLNSNH);
> 
> 
> With the 'Z' constraint, gcc generates:
> 
> 	str wzr, [x0]
> 
> without the 'Z' constraint, gcc generates:
> 
> 	mov	w1, 0
> 	str w1, [x0]
> 
> 
> I can work around the problem using the following patch:
> 
> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> index 045d93884164..93117519aed8 100644
> --- a/drivers/iommu/arm-smmu.c
> +++ b/drivers/iommu/arm-smmu.c
> @@ -59,6 +59,11 @@
>   
>   #include "arm-smmu-regs.h"
>   
> +static inline void qcom_writel(u32 val, volatile void __iomem *addr)
> +{
> +	asm volatile("str %w0, [%1]" : : "r" (val), "r" (addr));
> +}
> +
>   #define ARM_MMU500_ACTLR_CPRE		(1 << 1)
>   
>   #define ARM_MMU500_ACR_CACHE_LOCK	(1 << 26)
> @@ -422,7 +427,7 @@ static void __arm_smmu_tlb_sync(struct arm_smmu_device *smmu,
>   {
>   	unsigned int spin_cnt, delay;
>   
> -	writel_relaxed(0, sync);
> +	qcom_writel(0, sync);
>   	for (delay = 1; delay < TLB_LOOP_TIMEOUT; delay *= 2) {
>   		for (spin_cnt = TLB_SPIN_COUNT; spin_cnt > 0; spin_cnt--) {
>   			if (!(readl_relaxed(status) & sTLBGSTATUS_GSACTIVE))
> @@ -1760,8 +1765,8 @@ static void arm_smmu_device_reset(struct arm_smmu_device *smmu)
>   	}
>   
>   	/* Invalidate the TLB, just in case */
> -	writel_relaxed(0, gr0_base + ARM_SMMU_GR0_TLBIALLH);
> -	writel_relaxed(0, gr0_base + ARM_SMMU_GR0_TLBIALLNSNH);
> +	qcom_writel(0, gr0_base + ARM_SMMU_GR0_TLBIALLH);
> +	qcom_writel(0, gr0_base + ARM_SMMU_GR0_TLBIALLNSNH);
>   
>   	reg = readl_relaxed(ARM_SMMU_GR0_NS(smmu) + ARM_SMMU_GR0_sCR0);
>   
> 
> 
> 
> Can a quirk be used to work around the issue?
> Or can we just "pessimize" the 3 writes for everybody?
> (Might be cheaper than a test anyway)

If it really is just the SMMU driver which is affected, we can work 
around it for free (not counting the 'cost' of slightly-weird-looking 
code, of course). If the diff below works as expected, I'll write it up 
properly.

Robin.
----->8-----
diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
index 045d93884164..7ff29e33298f 100644
--- a/drivers/iommu/arm-smmu.c
+++ b/drivers/iommu/arm-smmu.c
@@ -422,7 +422,7 @@ static void __arm_smmu_tlb_sync(struct 
arm_smmu_device *smmu,
  {
  	unsigned int spin_cnt, delay;

-	writel_relaxed(0, sync);
+	writel_relaxed((unsigned long)sync, sync);
  	for (delay = 1; delay < TLB_LOOP_TIMEOUT; delay *= 2) {
  		for (spin_cnt = TLB_SPIN_COUNT; spin_cnt > 0; spin_cnt--) {
  			if (!(readl_relaxed(status) & sTLBGSTATUS_GSACTIVE))
@@ -681,7 +681,12 @@ static void arm_smmu_write_context_bank(struct 
arm_smmu_device *smmu, int idx)

  	/* Unassigned context banks only need disabling */
  	if (!cfg) {
-		writel_relaxed(0, cb_base + ARM_SMMU_CB_SCTLR);
+		/*
+		 * For Qualcomm reasons, we want to guarantee that we write a
+		 * zero from a register which is not WZR. Fortunately, the cfg
+		 * logic here plays right into our hands...
+		 */
+		writel_relaxed((unsigned long)cfg, cb_base + ARM_SMMU_CB_SCTLR);
  		return;
  	}

@@ -1760,8 +1765,8 @@ static void arm_smmu_device_reset(struct 
arm_smmu_device *smmu)
  	}

  	/* Invalidate the TLB, just in case */
-	writel_relaxed(0, gr0_base + ARM_SMMU_GR0_TLBIALLH);
-	writel_relaxed(0, gr0_base + ARM_SMMU_GR0_TLBIALLNSNH);
+	writel_relaxed(reg, gr0_base + ARM_SMMU_GR0_TLBIALLH);
+	writel_relaxed(reg, gr0_base + ARM_SMMU_GR0_TLBIALLNSNH);

  	reg = readl_relaxed(ARM_SMMU_GR0_NS(smmu) + ARM_SMMU_GR0_sCR0);