Re: [PATCH v6 4/6] swiotlb: Fix alignment checks when both allocation and DMA masks are present

[Will Deacon <will@kernel.org>]

On Mon, Mar 11, 2024 at 09:36:10PM +0000, Michael Kelley wrote:
> From: Petr Tesařík <petr@tesarici.cz>
> > On Fri,  8 Mar 2024 15:28:27 +0000
> > Will Deacon <will@kernel.org> wrote:
> > > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> > > index c20324fba814..c381a7ed718f 100644
> > > --- a/kernel/dma/swiotlb.c
> > > +++ b/kernel/dma/swiotlb.c
> > > @@ -981,8 +981,7 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
> > >  	dma_addr_t tbl_dma_addr =
> > >  		phys_to_dma_unencrypted(dev, pool->start) & boundary_mask;
> > >  	unsigned long max_slots = get_max_slots(boundary_mask);
> > > -	unsigned int iotlb_align_mask =
> > > -		dma_get_min_align_mask(dev) & ~(IO_TLB_SIZE - 1);
> > > +	unsigned int iotlb_align_mask = dma_get_min_align_mask(dev);
> > >  	unsigned int nslots = nr_slots(alloc_size), stride;
> > >  	unsigned int offset = swiotlb_align_offset(dev, orig_addr);
> > >  	unsigned int index, slots_checked, count = 0, i;
> > > @@ -993,6 +992,14 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
> > >  	BUG_ON(!nslots);
> > >  	BUG_ON(area_index >= pool->nareas);
> > >
> > > +	/*
> > > +	 * Ensure that the allocation is at least slot-aligned and update
> > > +	 * 'iotlb_align_mask' to ignore bits that will be preserved when
> > > +	 * offsetting into the allocation.
> > > +	 */
> > > +	alloc_align_mask |= (IO_TLB_SIZE - 1);
> > > +	iotlb_align_mask &= ~alloc_align_mask;
> > > +
> > 
> > I have started writing the KUnit test suite, and the results look
> > incorrect to me for this case.
> > 
> > I'm calling swiotlb_tbl_map_single() with:
> > 
> > * alloc_align_mask = 0xfff
> > * a device with min_align_mask = 0xfff
> > * the 12 lowest bits of orig_addr are 0xfa0
> > 
> > The min_align_mask becomes zero after the masking added by this patch,
> > and the 12 lowest bits of the returned address are 0x7a0, i.e. not
> > equal to 0xfa0.
> 
> The address returned by swiotlb_tbl_map_single() is the slot index
> converted to an address, plus the offset modulo the min_align_mask for
> the device.  The local variable "offset" in swiotlb_tbl_map_single()
> should be 0xfa0.  The slot index should be an even number to meet
> the alloc_align_mask requirement.  And the pool->start address should
> be at least page aligned, producing a page-aligned address *before* the
> offset is added. Can you debug which of these isn't true for the case
> you are seeing?

I was just looking into this, and I think the problem starts because
swiotlb_align_offset() doesn't return the offset modulo the min_align_mask,
but instead returns the offset *into the slot*:

	return addr & dma_get_min_align_mask(dev) & (IO_TLB_SIZE - 1);

so this presumably lops off bit 11 without adjusting the slot number.

I don't think swiotlb_find_slots() should be handling this though; it's
more about how swiotlb_tbl_map_single() puts the address back together
again.

> > In other words, the min_align_mask constraint is not honored. Of course,
> > given the above values, it is not possible to honor both min_align_mask
> > and alloc_align_mask. 
> 
> When orig_addr is specified and min_align_mask is set, alloc_align_mask
> governs the address of the _allocated_ space, which is not necessarily the
> returned physical address.  The min_align_mask may dictate some
> pre-padding of size "offset" within the allocated space, and the returned
> address is *after* that pre-padding.  In this way, both can be honored.

I agree, modulo the issue with the offset calculation.

Will