Messages in this thread | | | Subject | Re: [PATCH v3 1/2] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout | From | "Leizhen (ThunderTown)" <> | Date | Thu, 16 Aug 2018 12:11:43 +0800 |
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On 2018/8/16 2:08, John Garry wrote: > On 15/08/2018 14:00, Will Deacon wrote: >> On Wed, Aug 15, 2018 at 01:26:31PM +0100, Robin Murphy wrote: >>> On 15/08/18 11:23, 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. >>>> >>>> This is not a functional problem, just make the caller wait for a long >>>> time until TIMEOUT. It's rare to happen, because any other CMD_SYNCs >>>> during the waiting period will break it. >>>> >>>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com> >>>> --- >>>> drivers/iommu/arm-smmu-v3.c | 12 ++++++++---- >>>> 1 file changed, 8 insertions(+), 4 deletions(-) >>>> >>>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c >>>> index 1d64710..3f5c236 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 */ >>>> @@ -775,6 +775,11 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent) >>>> return 0; >>>> } >>>> >>>> +static inline void arm_smmu_cmdq_sync_set_msidata(u64 *cmd, u32 msidata) >>> >>> If we *are* going to go down this route then I think it would make sense to >>> move the msiaddr and CMDQ_SYNC_0_CS_MSI logic here as well; i.e. >>> arm_smmu_cmdq_build_cmd() always generates a "normal" SEV-based sync >>> command, then calling this guy would convert it to an MSI-based one. As-is, >>> having bits of mutually-dependent data handled across two separate places >>> just seems too messy and error-prone. >> >> Yeah, but I'd first like to see some number showing that doing all of this >> under the lock actually has an impact. > > Update: > > I tested this patch versus a modified version which builds the command under the queue spinlock (* below). From my testing there is a small difference: > > Setup: > Testing Single NVME card > fio 15 processes > No process pinning > > Average Results: > v3 patch read/r,w/write (IOPS): 301K/149K,149K/307K > Build under lock version read/r,w/write (IOPS): 304K/150K,150K/311K > > I don't know why it's better to build under the lock. We can test more.
I have analysed the assembly code, the memset will be optimized as Robin said to be "stp xzr, xzr, [x0]", and the switch..case is as below: ffff0000085e5744 <arm_smmu_cmdq_build_cmd>: ffff0000085e5744: a9007c1f stp xzr, xzr, [x0] //memset ffff0000085e5748: 39400023 ldrb w3, [x1] ffff0000085e574c: f9400002 ldr x2, [x0] ffff0000085e5750: aa020062 orr x2, x3, x2 ffff0000085e5754: f9000002 str x2, [x0] ffff0000085e5758: 39400023 ldrb w3, [x1] //ent->opcode ffff0000085e575c: 51000463 sub w3, w3, #0x1 ffff0000085e5760: 7101147f cmp w3, #0x45 ffff0000085e5764: 54000069 b.ls ffff0000085e5770 ffff0000085e5768: 12800023 mov w3, #0xfffffffe ffff0000085e576c: 1400000e b ffff0000085e57a4 ffff0000085e5770: b0003024 adrp x4, ffff000008bea000 ffff0000085e5774: 91096084 add x4, x4, #0x258 //static table in rodata ffff0000085e5778: 38634883 ldrb w3, [x4,w3,uxtw] //use ent->opcode as index ffff0000085e577c: 10000064 adr x4, ffff0000085e5788 ffff0000085e5780: 8b238883 add x3, x4, w3, sxtb #2 ffff0000085e5784: d61f0060 br x3 //jump to "case xxx:"
Actually, after apply the patch "inline arm_smmu_cmdq_build_cmd" sent by Robin, the memset and static table will be removed: ffff0000085e68a8: 94123207 bl ffff000008a730c4 <_raw_spin_lock_irqsave> ffff0000085e68ac: b9410ad5 ldr w21, [x22,#264] ffff0000085e68b0: aa0003fa mov x26, x0 ffff0000085e68b4: 110006b5 add w21, w21, #0x1 //++smmu->sync_nr ffff0000085e68b8: b9010ad5 str w21, [x22,#264] ffff0000085e68bc: b50005f3 cbnz x19, ffff0000085e6978 //if (ent->sync.msiaddr) ffff0000085e68c0: d28408c2 mov x2, #0x2046 ffff0000085e68c4: f2a1f802 movk x2, #0xfc0, lsl #16 //the constant part of CMD_SYNC ffff0000085e68c8: aa158042 orr x2, x2, x21, lsl #32 //or msidata ffff0000085e68cc: aa1603e0 mov x0, x22 //x0 = x22 = smmu ffff0000085e68d0: 910163a1 add x1, x29, #0x58 //x1 = the address of local variable "cmd" ffff0000085e68d4: f9002fa2 str x2, [x29,#88] //save cmd[0] ffff0000085e68d8: 927ec673 and x19, x19, #0xffffffffffffc ffff0000085e68dc: f90033b3 str x19, [x29,#96] //save cmd[1] ffff0000085e68e0: 97fffd0d bl ffff0000085e5d14 <arm_smmu_cmdq_insert_cmd>
So that, my patch v2 plus Robin's "inline arm_smmu_cmdq_build_cmd()" is a good choice.
But the assembly code of my patch v3, it seems that is still shorter than above: ffff0000085e695c: 9412320a bl ffff000008a73184 <_raw_spin_lock_irqsave> ffff0000085e6960: aa0003f6 mov x22, x0 ffff0000085e6964: b9410a62 ldr w2, [x19,#264] ffff0000085e6968: aa1303e0 mov x0, x19 ffff0000085e696c: f94023a3 ldr x3, [x29,#64] ffff0000085e6970: 910103a1 add x1, x29, #0x40 ffff0000085e6974: 11000442 add w2, w2, #0x1 //++smmu->sync_nr ffff0000085e6978: b9010a62 str w2, [x19,#264] ffff0000085e697c: b9005ba2 str w2, [x29,#88] ffff0000085e6980: aa028062 orr x2, x3, x2, lsl #32 ffff0000085e6984: f90023a2 str x2, [x29,#64] ffff0000085e6988: 97fffd58 bl ffff0000085e5ee8 <arm_smmu_cmdq_insert_cmd>
> > I suppose there is no justification to build the command outside the spinlock based on these results alone... > > Cheers, > John > > * Modified version: > static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu) > { > u64 cmd[CMDQ_ENT_DWORDS]; > unsigned long flags; > struct arm_smmu_cmdq_ent ent = { > .opcode = CMDQ_OP_CMD_SYNC, > .sync = { > .msiaddr = virt_to_phys(&smmu->sync_count), > }, > }; > > spin_lock_irqsave(&smmu->cmdq.lock, flags); > ent.sync.msidata = ++smmu->sync_nr; > arm_smmu_cmdq_build_cmd(cmd, &ent); > arm_smmu_cmdq_insert_cmd(smmu, cmd); > spin_unlock_irqrestore(&smmu->cmdq.lock, flags); > > return __arm_smmu_sync_poll_msi(smmu, ent.sync.msidata); > } > > >> Will >> >> . >> > > > > . >
-- Thanks! BestRegards
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