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

[Robin Murphy]

On 2024-03-12 8:52 am, Petr Tesařík wrote:
> On Mon, 11 Mar 2024 22:49:11 +0000
> Will Deacon <will@xxxxxxxxxx> wrote:
>
> > On Mon, Mar 11, 2024 at 09:36:10PM +0000, Michael Kelley wrote:
> > > From: Petr Tesařík <petr@xxxxxxxxxxx>
> > > > On Fri,  8 Mar 2024 15:28:27 +0000
> > > > Will Deacon <will@xxxxxxxxxx> 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.
>
> Yes. You will never see an offset bigger than IO_TLB_SIZE.
>
> > 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.
>
> *sigh*
>
> This is exactly what I tried to suggest here:
>
>    https://lore.kernel.org/linux-iommu/20240301180853.5ac20b27@xxxxxxxxxxxxxxxxxxxx/
>
> To which Robin Murphy replied:
>
> > That doesn't make sense - a caller asks to map some range of kernel
> > addresses and they get back a corresponding range of DMA addresses; they
> > cannot make any reasonable assumptions about DMA addresses *outside*
> > that range.
>
> It sounded like a misunderstanding back then already, but in light of
> the present findings, should I send the corresponding patch after all?

No, that comment was in reference to the idea of effectively forcing 
alloc_align_mask in order to honour min_align_mask - specifically that 
the reasoning given for it was spurious, but it's clear now it would 
also simply exacerbate this problem.

Simply put, if min_align_mask is specified alone, SWIOTLB can allocate a 
roughly-aligned range of slots such that the bounce offset is always 
less than IO_TLB_SIZE from the start of the allocation; if both 
min_align_mask and alloc_align_mask are specified, then the bounce 
offset may be larger than IO_TLB_SIZE, and SWIOTLB needs to be able to 
handle that correctly. There is still no benefit in forcing the latter 
case to happen more often than it needs to.

Thanks,
Robin.