how can we increase bounce buffer size from 1GB to 2GB

[aakarshcool15-crypto]

Is there any limitation of bounce buffer size?
in my system, I tried all permutations and combination to increase swiotlb pool size from 1GB to 2GB but it isn't increasing.

Is there any way to increase swiotlb pool? I need to run large network of 2GB.

Thanks in advance.

Thanks

[tlendacky]

You may be limited by the fact that SWIOTLB has to be below 4GB in memory. You can try this (old) patch to see if it allows you to allocate more memory using the swiotlb kernel parameter:

https://lore.kernel.org/lkml/20250429165330.2644155-1-jun.miao@intel.com/

However, if you have any devices in the guest that don't support 64-bit DMA, this may cause issues.

I haven't seen any conversation or discussion around this patch, though. But maybe you can reply to the thread and kick it to see if you can start a discussion given your requirements.

[aakarshcool15-crypto]

Hi, Thank you for your response

I applied the patch shared by you, but that didnt helped

I am working on 6.16.0-snp-guest-038d61fd6422+ kernel.

I am getting swiotlb buffer is full (sz: 2097152 bytes), total 524288 (slots), used 515875 (slots)

dmesg logs:
software IO TLB: mapped [mem 0x0000001fb9c00000-0x0000001ff9c00000] (1024MB)
software IO TLB: Memory encryption is active and system is using DMA bounce buffers

root@ccvm:# cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-6.16.0-snp-guest-038d61fd6422+ root=/dev/mapper/ubuntu--vg-ubuntu--lv ro console=ttyS0,115200n8,swiotlb=1048576,dynamic,force

These are my kernel changes

index 6267363e0189..73b9320c4a7d 100644
--- a/arch/x86/kernel/pci-dma.c
+++ b/arch/x86/kernel/pci-dma.c
@@ -61,7 +61,7 @@ static void __init pci_swiotlb_detect(void)
         */
        if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) {
                x86_swiotlb_enable = true;
-               x86_swiotlb_flags |= SWIOTLB_FORCE;
+               x86_swiotlb_flags |= SWIOTLB_ANY | SWIOTLB_FORCE;
        }
 }
 #else
diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index 3dae0f592063..908a36e40220 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -23,7 +23,7 @@ struct scatterlist;
  * must be a power of 2.  What is the appropriate value ?
  * The complexity of {map,unmap}_single is linearly dependent on this value.
  */
-#define IO_TLB_SEGSIZE 128
+#define IO_TLB_SEGSIZE 16384

 /*
  * log of the size of each IO TLB slab.  The number of slabs is command line
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index abcf3fa63a56..d740a1c445db 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -1190,7 +1190,7 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
        int cpu, i;
        int index;

-       if (alloc_size > IO_TLB_SEGSIZE * IO_TLB_SIZE)
+       if (alloc_size > (size_t)IO_TLB_SEGSIZE * IO_TLB_SIZE)
                return -1;

        cpu = raw_smp_processor_id();

Tried with IO_TLB_SEGSIZE 128 also but didnt worked

[aakarshcool15-crypto]

With this code it looks like we cannot increase swiotlb pool beyond 1GB.
https://elixir.bootlin.com/linux/v6.16/source/arch/x86/mm/mem_encrypt.c#L102
Is this true?

[tlendacky]

With this code it looks like we cannot increase swiotlb pool beyond 1GB. https://elixir.bootlin.com/linux/v6.16/source/arch/x86/mm/mem_encrypt.c#L102 Is this true?

No, if swiotlb has been specified, that should be used as an override.

On my guest with swiotlb=1048576:
[ 2.791579] software IO TLB: mapped [mem 0x00000007dcc00000-0x000000085cc00000] (2048MB)
[ 3.974736] software IO TLB: Memory encryption is active and system is using DMA bounce buffers

Looking at your command line, it should have space separators, but you have a comma so your swiotlb parameter is part of the console parameter:
console=ttyS0,115200n8,swiotlb=1048576,dynamic,force => console=ttyS0,115200n8 swiotlb=1048576

[aakarshcool15-crypto]

Thank you!!

WIth this patch https://lore.kernel.org/lkml/20250429165330.2644155-1-jun.miao@intel.com/#t and kernel command line swiotlb=1048576. I am able to increase swiotlb pool size to 2GB.

I will reply to this thread with my obervations.

Thank you once again!!