[PATCH 1/1] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout

[PATCH 1/1] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout

[Zhen Lei]

The condition "(int)(VAL - sync_idx) >= 0" to break loop in function
__arm_smmu_sync_poll_msi requires that sync_idx must be increased
monotonously according to the sequence of the CMDs in the cmdq.

But ".msidata = atomic_inc_return_relaxed(&smmu->sync_nr)" is not protected
by spinlock, so the following scenarios may appear:
cpu0			cpu1
msidata=0
			msidata=1
			insert cmd1
insert cmd0
			smmu execute cmd1
smmu execute cmd0
			poll timeout, because msidata=1 is overridden by
			cmd0, that means VAL=0, sync_idx=1.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
 drivers/iommu/arm-smmu-v3.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
index 1d64710..4810f61 100644
--- a/drivers/iommu/arm-smmu-v3.c
+++ b/drivers/iommu/arm-smmu-v3.c
@@ -566,7 +566,7 @@ struct arm_smmu_device {

 	int				gerr_irq;
 	int				combined_irq;
-	atomic_t			sync_nr;
+	u32				sync_nr;

 	unsigned long			ias; /* IPA */
 	unsigned long			oas; /* PA */
@@ -836,7 +836,6 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
 			cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_CS, CMDQ_SYNC_0_CS_SEV);
 		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSH, ARM_SMMU_SH_ISH);
 		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB);
-		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent->sync.msidata);
 		cmd[1] |= ent->sync.msiaddr & CMDQ_SYNC_1_MSIADDR_MASK;
 		break;
 	default:
@@ -947,7 +946,6 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
 	struct arm_smmu_cmdq_ent ent = {
 		.opcode = CMDQ_OP_CMD_SYNC,
 		.sync	= {
-			.msidata = atomic_inc_return_relaxed(&smmu->sync_nr),
 			.msiaddr = virt_to_phys(&smmu->sync_count),
 		},
 	};
@@ -955,6 +953,8 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
 	arm_smmu_cmdq_build_cmd(cmd, &ent);

 	spin_lock_irqsave(&smmu->cmdq.lock, flags);
+	ent.sync.msidata = ++smmu->sync_nr;
+	cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent.sync.msidata);
 	arm_smmu_cmdq_insert_cmd(smmu, cmd);
 	spin_unlock_irqrestore(&smmu->cmdq.lock, flags);

@@ -2179,7 +2179,6 @@ static int arm_smmu_init_structures(struct arm_smmu_device *smmu)
 {
 	int ret;

-	atomic_set(&smmu->sync_nr, 0);
 	ret = arm_smmu_init_queues(smmu);
 	if (ret)
 		return ret;
--
1.8.3

Re: [PATCH 1/1] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout

[Will Deacon]

Hi Thunder,

On Mon, Aug 06, 2018 at 08:31:29PM +0800, Zhen Lei wrote:
> The condition "(int)(VAL - sync_idx) >= 0" to break loop in function
> __arm_smmu_sync_poll_msi requires that sync_idx must be increased
> monotonously according to the sequence of the CMDs in the cmdq.
> 
> But ".msidata = atomic_inc_return_relaxed(&smmu->sync_nr)" is not protected
> by spinlock, so the following scenarios may appear:
> cpu0			cpu1
> msidata=0
> 			msidata=1
> 			insert cmd1
> insert cmd0
> 			smmu execute cmd1
> smmu execute cmd0
> 			poll timeout, because msidata=1 is overridden by
> 			cmd0, that means VAL=0, sync_idx=1.

Oh yuck, you're right! We probably want a CC stable on this. Did you see
this go wrong in practice?

One comment on your patch...

> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
>  drivers/iommu/arm-smmu-v3.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index 1d64710..4810f61 100644
> --- a/drivers/iommu/arm-smmu-v3.c
> +++ b/drivers/iommu/arm-smmu-v3.c
> @@ -566,7 +566,7 @@ struct arm_smmu_device {
> 
>  	int				gerr_irq;
>  	int				combined_irq;
> -	atomic_t			sync_nr;
> +	u32				sync_nr;
> 
>  	unsigned long			ias; /* IPA */
>  	unsigned long			oas; /* PA */
> @@ -836,7 +836,6 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
>  			cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_CS, CMDQ_SYNC_0_CS_SEV);
>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSH, ARM_SMMU_SH_ISH);
>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB);
> -		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent->sync.msidata);
>  		cmd[1] |= ent->sync.msiaddr & CMDQ_SYNC_1_MSIADDR_MASK;
>  		break;
>  	default:
> @@ -947,7 +946,6 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
>  	struct arm_smmu_cmdq_ent ent = {
>  		.opcode = CMDQ_OP_CMD_SYNC,
>  		.sync	= {
> -			.msidata = atomic_inc_return_relaxed(&smmu->sync_nr),
>  			.msiaddr = virt_to_phys(&smmu->sync_count),
>  		},
>  	};
> @@ -955,6 +953,8 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
>  	arm_smmu_cmdq_build_cmd(cmd, &ent);
> 
>  	spin_lock_irqsave(&smmu->cmdq.lock, flags);
> +	ent.sync.msidata = ++smmu->sync_nr;
> +	cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent.sync.msidata);

I really don't like splitting this out from building the rest of the
command. Can you just move the call to arm_smmu_cmdq_build_cmd into the
critical section, please?

Thanks,

Will

Re: [PATCH 1/1] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout

[Leizhen (ThunderTown)]

On 2018/8/8 18:12, Will Deacon wrote:
> Hi Thunder,
> 
> On Mon, Aug 06, 2018 at 08:31:29PM +0800, Zhen Lei wrote:
>> The condition "(int)(VAL - sync_idx) >= 0" to break loop in function
>> __arm_smmu_sync_poll_msi requires that sync_idx must be increased
>> monotonously according to the sequence of the CMDs in the cmdq.
>>
>> But ".msidata = atomic_inc_return_relaxed(&smmu->sync_nr)" is not protected
>> by spinlock, so the following scenarios may appear:
>> cpu0			cpu1
>> msidata=0
>> 			msidata=1
>> 			insert cmd1
>> insert cmd0
>> 			smmu execute cmd1
>> smmu execute cmd0
>> 			poll timeout, because msidata=1 is overridden by
>> 			cmd0, that means VAL=0, sync_idx=1.
> 
> Oh yuck, you're right! We probably want a CC stable on this. Did you see
> this go wrong in practice?
Just misreported and make the caller wait for a long time until TIMEOUT. It's
rare to happen, because any other CMD_SYNC during the waiting period will break
it.

> 
> One comment on your patch...
> 
>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>> ---
>>  drivers/iommu/arm-smmu-v3.c | 7 +++----
>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
>> index 1d64710..4810f61 100644
>> --- a/drivers/iommu/arm-smmu-v3.c
>> +++ b/drivers/iommu/arm-smmu-v3.c
>> @@ -566,7 +566,7 @@ struct arm_smmu_device {
>>
>>  	int				gerr_irq;
>>  	int				combined_irq;
>> -	atomic_t			sync_nr;
>> +	u32				sync_nr;
>>
>>  	unsigned long			ias; /* IPA */
>>  	unsigned long			oas; /* PA */
>> @@ -836,7 +836,6 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
>>  			cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_CS, CMDQ_SYNC_0_CS_SEV);
>>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSH, ARM_SMMU_SH_ISH);
>>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB);
>> -		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent->sync.msidata);
>>  		cmd[1] |= ent->sync.msiaddr & CMDQ_SYNC_1_MSIADDR_MASK;
>>  		break;
>>  	default:
>> @@ -947,7 +946,6 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
>>  	struct arm_smmu_cmdq_ent ent = {
>>  		.opcode = CMDQ_OP_CMD_SYNC,
>>  		.sync	= {
>> -			.msidata = atomic_inc_return_relaxed(&smmu->sync_nr),
>>  			.msiaddr = virt_to_phys(&smmu->sync_count),
>>  		},
>>  	};
>> @@ -955,6 +953,8 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
>>  	arm_smmu_cmdq_build_cmd(cmd, &ent);
>>
>>  	spin_lock_irqsave(&smmu->cmdq.lock, flags);
>> +	ent.sync.msidata = ++smmu->sync_nr;
>> +	cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent.sync.msidata);
> 
> I really don't like splitting this out from building the rest of the
> command. Can you just move the call to arm_smmu_cmdq_build_cmd into the
> critical section, please?
OK. I have considered that before, just worry it will increase the compition of spinlock.

In addition, I will append a optimization patch: the adjacent CMD_SYNCs, we only need one.

> 
> Thanks,
> 
> Will
> 
> .
> 

-- 
Thanks!
BestRegards

Re: [PATCH 1/1] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout

[Will Deacon]

On Thu, Aug 09, 2018 at 09:30:51AM +0800, Leizhen (ThunderTown) wrote:
> On 2018/8/8 18:12, Will Deacon wrote:
> > On Mon, Aug 06, 2018 at 08:31:29PM +0800, Zhen Lei wrote:
> >> The condition "(int)(VAL - sync_idx) >= 0" to break loop in function
> >> __arm_smmu_sync_poll_msi requires that sync_idx must be increased
> >> monotonously according to the sequence of the CMDs in the cmdq.
> >>
> >> But ".msidata = atomic_inc_return_relaxed(&smmu->sync_nr)" is not protected
> >> by spinlock, so the following scenarios may appear:
> >> cpu0			cpu1
> >> msidata=0
> >> 			msidata=1
> >> 			insert cmd1
> >> insert cmd0
> >> 			smmu execute cmd1
> >> smmu execute cmd0
> >> 			poll timeout, because msidata=1 is overridden by
> >> 			cmd0, that means VAL=0, sync_idx=1.
> > 
> > Oh yuck, you're right! We probably want a CC stable on this. Did you see
> > this go wrong in practice?
> Just misreported and make the caller wait for a long time until TIMEOUT. It's
> rare to happen, because any other CMD_SYNC during the waiting period will break
> it.

Thanks. Please mention that in the commit message, because I think it's
useful to know.

> >> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> >> ---
> >>  drivers/iommu/arm-smmu-v3.c | 7 +++----
> >>  1 file changed, 3 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> >> index 1d64710..4810f61 100644
> >> --- a/drivers/iommu/arm-smmu-v3.c
> >> +++ b/drivers/iommu/arm-smmu-v3.c
> >> @@ -566,7 +566,7 @@ struct arm_smmu_device {
> >>
> >>  	int				gerr_irq;
> >>  	int				combined_irq;
> >> -	atomic_t			sync_nr;
> >> +	u32				sync_nr;
> >>
> >>  	unsigned long			ias; /* IPA */
> >>  	unsigned long			oas; /* PA */
> >> @@ -836,7 +836,6 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
> >>  			cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_CS, CMDQ_SYNC_0_CS_SEV);
> >>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSH, ARM_SMMU_SH_ISH);
> >>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB);
> >> -		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent->sync.msidata);
> >>  		cmd[1] |= ent->sync.msiaddr & CMDQ_SYNC_1_MSIADDR_MASK;
> >>  		break;
> >>  	default:
> >> @@ -947,7 +946,6 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
> >>  	struct arm_smmu_cmdq_ent ent = {
> >>  		.opcode = CMDQ_OP_CMD_SYNC,
> >>  		.sync	= {
> >> -			.msidata = atomic_inc_return_relaxed(&smmu->sync_nr),
> >>  			.msiaddr = virt_to_phys(&smmu->sync_count),
> >>  		},
> >>  	};
> >> @@ -955,6 +953,8 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
> >>  	arm_smmu_cmdq_build_cmd(cmd, &ent);
> >>
> >>  	spin_lock_irqsave(&smmu->cmdq.lock, flags);
> >> +	ent.sync.msidata = ++smmu->sync_nr;
> >> +	cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent.sync.msidata);
> > 
> > I really don't like splitting this out from building the rest of the
> > command. Can you just move the call to arm_smmu_cmdq_build_cmd into the
> > critical section, please?
> OK. I have considered that before, just worry it will increase the
> compition of spinlock.

If you can provide numbers showing that it's a problem, then we could add
a helper function e.g. arm_smmu_cmdq_sync_set_msidata(arm_smmu_cmdq_ent *cmd)

> In addition, I will append a optimization patch: the adjacent CMD_SYNCs,
> we only need one.

Ok, but please keep them separate, since I don't want to fix up fixes and
optimisations.

Thanks,

Will

Re: [PATCH 1/1] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout

[Leizhen (ThunderTown)]

On 2018/8/9 16:49, Will Deacon wrote:
> On Thu, Aug 09, 2018 at 09:30:51AM +0800, Leizhen (ThunderTown) wrote:
>> On 2018/8/8 18:12, Will Deacon wrote:
>>> On Mon, Aug 06, 2018 at 08:31:29PM +0800, Zhen Lei wrote:
>>>> The condition "(int)(VAL - sync_idx) >= 0" to break loop in function
>>>> __arm_smmu_sync_poll_msi requires that sync_idx must be increased
>>>> monotonously according to the sequence of the CMDs in the cmdq.
>>>>
>>>> But ".msidata = atomic_inc_return_relaxed(&smmu->sync_nr)" is not protected
>>>> by spinlock, so the following scenarios may appear:
>>>> cpu0			cpu1
>>>> msidata=0
>>>> 			msidata=1
>>>> 			insert cmd1
>>>> insert cmd0
>>>> 			smmu execute cmd1
>>>> smmu execute cmd0
>>>> 			poll timeout, because msidata=1 is overridden by
>>>> 			cmd0, that means VAL=0, sync_idx=1.
>>>
>>> Oh yuck, you're right! We probably want a CC stable on this. Did you see
>>> this go wrong in practice?
>> Just misreported and make the caller wait for a long time until TIMEOUT. It's
>> rare to happen, because any other CMD_SYNC during the waiting period will break
>> it.
> 
> Thanks. Please mention that in the commit message, because I think it's
> useful to know.

OK.

> 
>>>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>>>> ---
>>>>  drivers/iommu/arm-smmu-v3.c | 7 +++----
>>>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
>>>> index 1d64710..4810f61 100644
>>>> --- a/drivers/iommu/arm-smmu-v3.c
>>>> +++ b/drivers/iommu/arm-smmu-v3.c
>>>> @@ -566,7 +566,7 @@ struct arm_smmu_device {
>>>>
>>>>  	int				gerr_irq;
>>>>  	int				combined_irq;
>>>> -	atomic_t			sync_nr;
>>>> +	u32				sync_nr;
>>>>
>>>>  	unsigned long			ias; /* IPA */
>>>>  	unsigned long			oas; /* PA */
>>>> @@ -836,7 +836,6 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
>>>>  			cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_CS, CMDQ_SYNC_0_CS_SEV);
>>>>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSH, ARM_SMMU_SH_ISH);
>>>>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB);
>>>> -		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent->sync.msidata);
>>>>  		cmd[1] |= ent->sync.msiaddr & CMDQ_SYNC_1_MSIADDR_MASK;
>>>>  		break;
>>>>  	default:
>>>> @@ -947,7 +946,6 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
>>>>  	struct arm_smmu_cmdq_ent ent = {
>>>>  		.opcode = CMDQ_OP_CMD_SYNC,
>>>>  		.sync	= {
>>>> -			.msidata = atomic_inc_return_relaxed(&smmu->sync_nr),
>>>>  			.msiaddr = virt_to_phys(&smmu->sync_count),
>>>>  		},
>>>>  	};
>>>> @@ -955,6 +953,8 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
>>>>  	arm_smmu_cmdq_build_cmd(cmd, &ent);
>>>>
>>>>  	spin_lock_irqsave(&smmu->cmdq.lock, flags);
>>>> +	ent.sync.msidata = ++smmu->sync_nr;
>>>> +	cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent.sync.msidata);
>>>
>>> I really don't like splitting this out from building the rest of the
>>> command. Can you just move the call to arm_smmu_cmdq_build_cmd into the
>>> critical section, please?
>> OK. I have considered that before, just worry it will increase the
>> compition of spinlock.
> 
> If you can provide numbers showing that it's a problem, then we could add
> a helper function e.g. arm_smmu_cmdq_sync_set_msidata(arm_smmu_cmdq_ent *cmd)

The performance data from my current test envirnoment is not stable now, I'm
trying to find anothor one. So I want to leave this problem for the time being.
If it'a problem, I will send a new patch.

> 
>> In addition, I will append a optimization patch: the adjacent CMD_SYNCs,
>> we only need one.
> 
> Ok, but please keep them separate, since I don't want to fix up fixes and
> optimisations.

OK

> 
> Thanks,
> 
> Will
> 
> .
> 

-- 
Thanks!
BestRegards