Re: [PATCH kernel] x86/swiotlb/amd: Half the size if allocation failed

[Alexey Kardashevskiy <aik@xxxxxxx>]

On 28/10/22 03:46, Thadeu Lima de Souza Cascardo wrote:
> On Thu, Oct 27, 2022 at 04:26:07PM +1100, Alexey Kardashevskiy wrote:
> > At the moment the AMD encrypted platform reserves 6% of RAM for SWIOTLB
> > or 1GB, whichever is less. However it is possible that there is no block
> > big enough in the low memory which make SWIOTLB allocation fail and
> > the kernel continues without DMA. In such case a VM hangs on DMA.
> >
> > This divides the size in half and tries again reusing the existing
> > remapping logic.
> >
> > This updates default_nslabs on successful allocation which looks like
> > an oversight as not doing so should have broken callers of
> > swiotlb_size_or_default().
> >
> > Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
>
> Should this have a
> Fixes: e998879d4fb7 ("x86,swiotlb: Adjust SWIOTLB bounce buffer size for SEV guests")
> ?


Well, the problem was there before that patch, the allocation failure 
was not handled while remap failure was. e998879d4fb7 just made it 
easier to see. But still worth mentioning I guess... Thanks,


> Cascardo.
>
> > --
> >
> > I hit the problem with
> > QEMU's "-m 16819M" where SWIOTLB was adjusted to
> > 0x7e200 == 1,058,013,184 (slightly less than 1GB) while
> > 0x7e180 still worked.
> >
> > With guest errors enabled, there are many unassigned accesses from
> > virtio.
> >
> > ---
> >   kernel/dma/swiotlb.c | 20 +++++++++++++-------
> >   1 file changed, 13 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> > index 339a990554e7..d28c294320fd 100644
> > --- a/kernel/dma/swiotlb.c
> > +++ b/kernel/dma/swiotlb.c
> > @@ -338,21 +338,27 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
> >   	else
> >   		tlb = memblock_alloc_low(bytes, PAGE_SIZE);
> >   	if (!tlb) {
> > -		pr_warn("%s: failed to allocate tlb structure\n", __func__);
> > -		return;
> > -	}
> > -
> > -	if (remap && remap(tlb, nslabs) < 0) {
> > +		pr_warn("%s: Failed to allocate %zu bytes tlb structure\n",
> > +			__func__, bytes);
> > +	} else if (remap && remap(tlb, nslabs) < 0) {
> >   		memblock_free(tlb, PAGE_ALIGN(bytes));
> > +		tlb = NULL;
> > +		pr_warn("%s: Failed to remap %zu bytes\n", __func__, bytes);
> > +	}
> >   
> > +	if (!tlb) {
> >   		nslabs = ALIGN(nslabs >> 1, IO_TLB_SEGSIZE);
> >   		if (nslabs >= IO_TLB_MIN_SLABS)
> >   			goto retry;
> > -
> > -		pr_warn("%s: Failed to remap %zu bytes\n", __func__, bytes);
> >   		return;
> >   	}
> >   
> > +	if (default_nslabs != nslabs) {
> > +		pr_info("SWIOTLB bounce buffer size adjusted %lu -> %lu slabs",
> > +			default_nslabs, nslabs);
> > +		default_nslabs = nslabs;
> > +	}
> > +
> >   	alloc_size = PAGE_ALIGN(array_size(sizeof(*mem->slots), nslabs));
> >   	mem->slots = memblock_alloc(alloc_size, PAGE_SIZE);
> >   	if (!mem->slots) {
> > --
> > 2.37.3

--
Alexey