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

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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",



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