Re: [RFC PATCH v3 09/12] iommu/io-pgtable-arm: Implement arm_lpae_unmap_pages()

[isaacm@codeaurora.org]

On 2021-04-06 05:15, Will Deacon wrote:
> On Mon, Apr 05, 2021 at 12:11:09PM -0700, Isaac J. Manjarres wrote:
>> Implement the unmap_pages() callback for the ARM LPAE io-pgtable
>> format.
>> 
>> Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
>> Suggested-by: Will Deacon <will@kernel.org>
>> ---
>>  drivers/iommu/io-pgtable-arm.c | 124 
>> +++++++++++++++++++++++++++------
>>  1 file changed, 104 insertions(+), 20 deletions(-)
> 
> [...]
> 
>> +static size_t arm_lpae_unmap_pages(struct io_pgtable_ops *ops, 
>> unsigned long iova,
>> +				   size_t pgsize, size_t pgcount,
>> +				   struct iommu_iotlb_gather *gather)
>> +{
>> +	struct arm_lpae_io_pgtable *data = io_pgtable_ops_to_data(ops);
>> +	struct io_pgtable_cfg *cfg = &data->iop.cfg;
>> +	arm_lpae_iopte *ptep = data->pgd;
>> +	long iaext = (s64)iova >> cfg->ias;
>> +	size_t unmapped = 0, unmapped_page;
>> +	int last_lvl;
>> +	size_t table_size, pages, tbl_offset, max_entries;
>> +
>> +	if (WARN_ON(!pgsize || (pgsize & cfg->pgsize_bitmap) != pgsize || 
>> !pgcount))
>> +		return 0;
>> +
>> +	if (cfg->quirks & IO_PGTABLE_QUIRK_ARM_TTBR1)
>> +		iaext = ~iaext;
>> +	if (WARN_ON(iaext))
>> +		return 0;
>> +
>> +	/*
>> +	 * Calculating the page table size here helps avoid situations where
>> +	 * a page range that is being unmapped may be mapped at the same 
>> level
>> +	 * but not mapped by the same tables. Allowing such a scenario to
>> +	 * occur can complicate the logic in arm_lpae_split_blk_unmap().
>> +	 */
>> +	last_lvl = ARM_LPAE_BLOCK_SIZE_LVL(pgsize, data);
>> +
>> +	if (last_lvl == data->start_level)
>> +		table_size = ARM_LPAE_PGD_SIZE(data);
>> +	else
>> +		table_size = ARM_LPAE_GRANULE(data);
>> +
>> +	max_entries = table_size / sizeof(*ptep);
>> +
>> +	while (pgcount) {
>> +		tbl_offset = ARM_LPAE_LVL_IDX(iova, last_lvl, data);
>> +		pages = min_t(size_t, pgcount, max_entries - tbl_offset);
>> +		unmapped_page = __arm_lpae_unmap(data, gather, iova, pgsize,
>> +						 pages, data->start_level,
>> +						 ptep);
>> +		if (!unmapped_page)
>> +			break;
>> +
>> +		unmapped += unmapped_page;
>> +		iova += unmapped_page;
>> +		pgcount -= pages;
>> +	}
> 
> Robin has some comments on the first version of this patch, and I
> don't think you
> addressed them:
> 
> https://lore.kernel.org/r/b93fa0b1-e2a4-1aad-8b88-4d0dfecdfef7@arm.com
> 
> I'm inclined to agree that iterating here doesn't make a lot of sense 
> --
> we've already come back out of the page-table walk, so I think we 
> should
> just return to the caller (who is well prepared to handle a partial 
> unmap).
> Same for the map side of things.
> 
> If we get numbers showing that this is causing a performance issue, 
> then
> we should rework the page-table code to handle this at the lower level
> (because I doubt the loop that you have is really worse than returning 
> to
> the caller anyway).
> 
Sorry, I seem to have overlooked those comments.

I will go ahead and address them. I think it might be ideal to try to do
as much work as possible in the io-pgtable level, so as to minimize the 
number
of indirect calls incurred by jumping back and forth between iommu fwk, 
iommu
driver, and io-pgtable code.

Perhaps we can try something like how the linear mappings are created on 
arm64 i.e.
on the previous level, we can determine how many pages can be unmapped 
in one page table
in one iteration, and on the subsequent iterations, we can tackle 
another page table at
the lower level. Looking at the code, it doesn't seem too difficult to 
add this in. Thoughts?

Thanks,
Isaac

> Will
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