Re: [PATCH 0/2] x86/hyperv/Swiotlb: Add swiotlb_alloc_from_low_pages switch

[Tianyu Lan]

On 2/2/2022 4:12 PM, Christoph Hellwig wrote:
> I think this interface is a little too hacky.  In the end all the
> non-trusted hypervisor schemes (including the per-device swiotlb one)
> can allocate the memory from everywhere and want for force use of
> swiotlb.  I think we need some kind of proper interface for that instead
> of setting all kinds of global variables.

Hi Christoph:
     Thanks for your review. I draft the following patch to export a
interface swiotlb_set_alloc_from_low_pages(). Could you have a look
whether this looks good for you.

diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index f6c3638255d5..2b4f92668bc7 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -39,6 +39,7 @@ enum swiotlb_force {
 extern void swiotlb_init(int verbose);
 int swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose);
 unsigned long swiotlb_size_or_default(void);
+void swiotlb_set_alloc_from_low_pages(bool low);
 extern int swiotlb_late_init_with_tbl(char *tlb, unsigned long nslabs);
 extern int swiotlb_late_init_with_default_size(size_t default_size);
 extern void __init swiotlb_update_mem_attributes(void);
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index f1e7ea160b43..62bf8b5cc3e4 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -73,6 +73,8 @@ enum swiotlb_force swiotlb_force;

 struct io_tlb_mem io_tlb_default_mem;

+static bool swiotlb_alloc_from_low_pages = true;
+
 phys_addr_t swiotlb_unencrypted_base;

 /*
@@ -116,6 +118,11 @@ void swiotlb_set_max_segment(unsigned int val)
                max_segment = rounddown(val, PAGE_SIZE);
 }

+void swiotlb_set_alloc_from_low_pages(bool low)
+{
+       swiotlb_alloc_from_low_pages = low;
+}
+
 unsigned long swiotlb_size_or_default(void)
 {
        return default_nslabs << IO_TLB_SHIFT;
@@ -284,8 +291,15 @@ swiotlb_init(int verbose)
        if (swiotlb_force == SWIOTLB_NO_FORCE)
                return;

-       /* Get IO TLB memory from the low pages */
-       tlb = memblock_alloc_low(bytes, PAGE_SIZE);
+       /*
+        * Get IO TLB memory from the low pages if 
swiotlb_alloc_from_low_pages
+        * is set.
+        */
+       if (swiotlb_alloc_from_low_pages)
+               tlb = memblock_alloc_low(bytes, PAGE_SIZE);
+       else
+               tlb = memblock_alloc(bytes, PAGE_SIZE);
+
        if (!tlb)
                goto fail;
        if (swiotlb_init_with_tbl(tlb, default_nslabs, verbose))