Re: [PATCH v5 01/17] iommu/arm-smmu-v3: Make STE programming independent of the callers

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

On 15/02/2024 4:01 pm, Jason Gunthorpe wrote:
> On Thu, Feb 15, 2024 at 01:49:53PM +0000, Will Deacon wrote:
>> Hi Jason,
>>
>> On Tue, Feb 06, 2024 at 11:12:38AM -0400, Jason Gunthorpe wrote:
>>> As the comment in arm_smmu_write_strtab_ent() explains, this routine has
>>> been limited to only work correctly in certain scenarios that the caller
>>> must ensure. Generally the caller must put the STE into ABORT or BYPASS
>>> before attempting to program it to something else.
>>
>> This is looking pretty good now, but I have a few comments inline.
> 
> Ok
> 
>>> @@ -48,6 +48,21 @@ enum arm_smmu_msi_index {
>>>   	ARM_SMMU_MAX_MSIS,
>>>   };
>>>   
>>> +struct arm_smmu_entry_writer_ops;
>>> +struct arm_smmu_entry_writer {
>>> +	const struct arm_smmu_entry_writer_ops *ops;
>>> +	struct arm_smmu_master *master;
>>> +};
>>> +
>>> +struct arm_smmu_entry_writer_ops {
>>> +	unsigned int num_entry_qwords;
>>> +	__le64 v_bit;
>>> +	void (*get_used)(const __le64 *entry, __le64 *used);
>>> +	void (*sync)(struct arm_smmu_entry_writer *writer);
>>> +};
>>
>> Can we avoid the indirection for now, please? I'm sure we'll want it later
>> when you extend this to CDs, but for the initial support it just makes it
>> more difficult to follow the flow. Should be a trivial thing to drop, I
>> hope.
> 
> We can.

Ack, the abstraction is really hard to follow, and much of that seems 
entirely self-inflicted in the amount of recalculating information which 
was in-context in a previous step but then thrown away. And as best I 
can tell I think it will still end up doing more CFGIs than needed.

Keeping a single monolithic check-and-update function will be *so* much 
easier to understand and maintain. As far as CDs go, anything we might 
reasonably want to change in a live CD is all in the first word so I 
don't see any value in attempting to generalise further on that side of 
things. Maybe arm_smmu_write_ctx_desc() could stand to be a bit 
prettier, but honestly I don't think it's too bad as-is.

>>> +static void arm_smmu_get_ste_used(const __le64 *ent, __le64 *used_bits)
>>>   {
>>> +	unsigned int cfg = FIELD_GET(STRTAB_STE_0_CFG, le64_to_cpu(ent[0]));
>>> +
>>> +	used_bits[0] = cpu_to_le64(STRTAB_STE_0_V);
>>> +	if (!(ent[0] & cpu_to_le64(STRTAB_STE_0_V)))
>>> +		return;
>>> +
>>> +	/*
>>> +	 * See 13.5 Summary of attribute/permission configuration fields for the
>>> +	 * SHCFG behavior. It is only used for BYPASS, including S1DSS BYPASS,
>>> +	 * and S2 only.
>>> +	 */
>>> +	if (cfg == STRTAB_STE_0_CFG_BYPASS ||
>>> +	    cfg == STRTAB_STE_0_CFG_S2_TRANS ||
>>> +	    (cfg == STRTAB_STE_0_CFG_S1_TRANS &&
>>> +	     FIELD_GET(STRTAB_STE_1_S1DSS, le64_to_cpu(ent[1])) ==
>>> +		     STRTAB_STE_1_S1DSS_BYPASS))
>>> +		used_bits[1] |= cpu_to_le64(STRTAB_STE_1_SHCFG);
>>
>> Huh, SHCFG is really getting in the way here, isn't it?
> 
> I wouldn't say that.. It is just a complicated bit of the spec. One of
> the things we recently did was to audit all the cache settings and, at
> least, we then realized that SHCFG was being subtly used by S2 as
> well..

Yeah, that really shouldn't be subtle; incoming attributes are replaced 
by S1 translation, thus they are relevant to not-S1 configs.

I think it's likely to be significantly more straightforward to give up 
on the switch statement and jump straight into the more architectural 
paradigm at this level, e.g.

	// Stage 1
	if (cfg & BIT(0)) {
		...
	} else {
		...
	}
	// Stage 2
	if (cfg & BIT(1)) {
		...
	} else {
		...
	}

Thanks,
Robin.