Re: [PATCH 12/15] swiotlb: provide swiotlb_init variants that remap the buffer

Boris Ostrovsky
2022-03-16T00:41:01Z

On 3/15/22 2:36 AM, Christoph Hellwig wrote:

> @@ -271,12 +273,23 @@ void __init swiotlb_init(bool addressing_limit, unsigned int flags)
> * allow to pick a location everywhere for hypervisors with guest
> * memory encryption.
> */
> +retry:
> + bytes = PAGE_ALIGN(default_nslabs << IO_TLB_SHIFT);
> if (flags & SWIOTLB_ANY)
> tlb = memblock_alloc(bytes, PAGE_SIZE);
> else
> tlb = memblock_alloc_low(bytes, PAGE_SIZE);
> if (!tlb)
> goto fail;
> + if (remap && remap(tlb, nslabs) < 0) {
> + memblock_free(tlb, PAGE_ALIGN(bytes));
> +
> + if (nslabs <= IO_TLB_MIN_SLABS)
> + panic("%s: Failed to remap %zu bytes\n",
> + __func__, bytes);
> + nslabs = max(1024UL, ALIGN(nslabs >> 1, IO_TLB_SEGSIZE));

I spoke with Konrad (who wrote the original patch --- f4b2f07b2ed9b469ead87e06fc2fc3d12663a725) and apparently the reason for 2MB was to optimize for Xen's slab allocator, it had nothing to do with IO_TLB_MIN_SLABS. Since this is now common code we should not expose Xen-specific optimizations here and smaller values will still work so IO_TLB_MIN_SLABS is fine.

I think this should be mentioned in the commit message though, probably best in the next patch where you switch to this code.

As far as the hunk above, I don't think we need the max() here: with IO_TLB_MIN_SLABS being 512 we may get stuck in an infinite loop. Something like

nslabs = ALIGN(nslabs >> 1, IO_TLB_SEGSIZE);
if (nslabs <= IO_TLB_MIN_SLABS)
panic()

should be sufficient.

> + goto retry;
> + }
> if (swiotlb_init_with_tbl(tlb, default_nslabs, flags))
> goto fail_free_mem;
> return;
> @@ -287,12 +300,18 @@ void __init swiotlb_init(bool addressing_limit, unsigned int flags)
> pr_warn("Cannot allocate buffer");
> }
>
> +void __init swiotlb_init(bool addressing_limit, unsigned int flags)
> +{
> + return swiotlb_init_remap(addressing_limit, flags, NULL);
> +}
> +
> /*
> * Systems with larger DMA zones (those that don't support ISA) can
> * initialize the swiotlb later using the slab allocator if needed.
> * This should be just like above, but with some error catching.
> */
> -int swiotlb_init_late(size_t size, gfp_t gfp_mask)
> +int swiotlb_init_late(size_t size, gfp_t gfp_mask,
> + int (*remap)(void *tlb, unsigned long nslabs))
> {
> unsigned long nslabs = ALIGN(size >> IO_TLB_SHIFT, IO_TLB_SEGSIZE);
> unsigned long bytes;
> @@ -303,6 +322,7 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask)
> if (swiotlb_force_disable)
> return 0;
>
> +retry:
> order = get_order(nslabs << IO_TLB_SHIFT);
> nslabs = SLABS_PER_PAGE << order;
> bytes = nslabs << IO_TLB_SHIFT;
> @@ -317,6 +337,16 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask)
>
> if (!vstart)
> return -ENOMEM;
> + if (remap)
> + rc = remap(vstart, nslabs);
> + if (rc) {
> + free_pages((unsigned long)vstart, order);
> +
> + if (IO_TLB_MIN_SLABS <= 1024)
> + return rc;
> + nslabs = max(1024UL, ALIGN(nslabs >> 1, IO_TLB_SEGSIZE));

Same here. (The 'if' check above is wrong anyway).

Patches 13 and 14 look good.

-boris

> + goto retry;
> + }
>
> if (order != get_order(bytes)) {
> pr_warn("only able to allocate %ld MB\n",