Re: [PATCH v5 05/27] iommu/arm-smmu-v3: Make CD programming use arm_smmu_write_entry()

[Mostafa Saleh <smostafa@google.com>]

On Tue, Mar 26, 2024 at 07:27:58PM -0300, Jason Gunthorpe wrote:
> On Tue, Mar 26, 2024 at 07:12:53PM +0000, Mostafa Saleh wrote:
> > On Tue, Mar 26, 2024 at 03:30:55PM -0300, Jason Gunthorpe wrote:
> > > On Sat, Mar 23, 2024 at 01:02:15PM +0000, Mostafa Saleh wrote:
> > > > > +static void arm_smmu_get_cd_used(const __le64 *ent, __le64 *used_bits)
> > > > > +{
> > > > > +	used_bits[0] = cpu_to_le64(CTXDESC_CD_0_V);
> > > > > +	if (!(ent[0] & cpu_to_le64(CTXDESC_CD_0_V)))
> > > > > +		return;
> > > > > +	memset(used_bits, 0xFF, sizeof(struct arm_smmu_cd));
> > > > 
> > > > This is a slightly different approach than what the driver does for STEs,
> > > > where it explicitly sets the used bits. Is there a reason for that?
> > > 
> > > It is just more compact this way
> > 
> > IMHO, it seems too much to have this mechanism for CDs for just one
> > SVA case, but I'll need to go through the whole seires first to make
> > sure I am not missing anything.
> 
> It is pretty ugly if you try to do it that way. You still need to
> create some ops because the entry_set should be re-used (I mean I
> guess you could copy it as well). Then you have to open code the
> logic. And then the EPD0 path is somewhat fragile. Something sort of
> like this:
> 
> void arm_smmu_write_cd_entry(struct arm_smmu_master *master, int ssid,
> 			     struct arm_smmu_cd *cdptr,
> 			     const struct arm_smmu_cd *target)
> {
> 	bool target_valid = target->data[0] & cpu_to_le64(CTXDESC_CD_0_V);
> 	bool cur_valid = cdptr->data[0] & cpu_to_le64(CTXDESC_CD_0_V);
> 	struct arm_smmu_cd_writer cd_writer = {
> 		.writer = {
> 			.ops = &arm_smmu_cd_writer_ops,
> 			.master = master,
> 		},
> 		.ssid = ssid,
> 	};
> 
> 	if (ssid != IOMMU_NO_PASID && cur_valid != target_valid) {
> 		if (cur_valid)
> 			master->cd_table.used_ssids--;
> 		else
> 			master->cd_table.used_ssids++;
> 	}
> 
> 	/* Force a V=0/V=1 update*/
> 	__le64 update = target[0] & ~cpu_to_le64(CTXDESC_CD_0_V);
> 	entry_set(&cd_writer.writer, cdptr->data, &update, 0, 1);
> 	entry_set(&cd_writer.writer, cdptr->data, target->data, 1, NUM_ENTRY_QWORDS - 1);
> 	entry_set(&cd_writer.writer, cdptr->data, target->data, 0, 1);
> }
> 
> void arm_smmu_write_cd_entry_epd0(struct arm_smmu_master *master, int ssid,
> 				  struct arm_smmu_cd *cdptr,
> 				  const struct arm_smmu_cd *target)
> {
> 	struct arm_smmu_cd_writer cd_writer = {
> 		.writer = {
> 			.ops = &arm_smmu_cd_writer_ops,
> 			.master = master,
> 		},
> 		.ssid = ssid,
> 	};
> 
> 	/*
> 	 * Target must the EPD0 = 1 version of the existing CD entry, caller
> 	 * must enforce it. Assume used_ssids doesn't need updating
> 	 * for this reason.
> 	 */
> 	/* Update EPD0 */
> 	entry_set(&cd_writer.writer, cdptr->data, target->data, 0, 1);
> 	/* Update everthing else */
> 	entry_set(&cd_writer.writer, cdptr->data, target->data, 0, NUM_ENTRY_QWORDS - 1);
> }
> 
> IMOH, at this point it is saner to have just implemented the used
> function and use the mechanism robustly. Less special cases, less
> fragility, less duplication.
> 

But that adds extra cost of adding ops, indirection, modifying STE
code..., for a case that is not common, so I think special casing it
is actually better for readability and maintainability.
But again, I need to finish going through the series to get the
full context.

Thanks,
Mostafa