RFC: How to make max_bounce_buffer_size configurable for address_space_memory

[Jonathan Cameron via <qemu-devel@nongnu.org>]

Hi All,

This closely related to Mattias' work to resolve bounce buffer limitations for
PCI memory spaces.
https://lore.kernel.org/qemu-devel/20240819135455.2957406-1-mnissler@rivosinc.com/

For CXL memory, due to the way interleave memory is emulated we end
up with the same problem with concurrent virtio mappings into IOMEM but
in this case they are in the address_space_memory.  In my tests
virtio-blk tends to fail as a result.  Note whilst QEMU sees this as
IOMEM, in the host it's just 'normal RAM' (be it with terrible performance :)

Currently I'm carrying the hack (obviously I never checked how much
space I actually needed as it's unlikely to be that much :)

diff --git a/system/physmem.c b/system/physmem.c
index e97de3ef65cf8105b030a44e7a481b1679f86b53..fd0848c1d5b982c3255a7c6c8c1f22b32c86b85a 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -2787,6 +2787,7 @@ static void memory_map_init(void)
     memory_region_init(system_memory, NULL, "system", UINT64_MAX);
     address_space_init(&address_space_memory, system_memory, "memory");
 
+    address_space_memory.max_bounce_buffer_size = 1024 * 1024 * 1024;
     system_io = g_malloc(sizeof(*system_io));
     memory_region_init_io(system_io, NULL, &unassigned_io_ops, NULL, "io",
                           65536);


Assuming people are amenable to making this configurable with a parameter
like x-max-bounce-buffer-size (from Mattias' set) how would people like
that to be configured?  The address_space_init() call is
fairly early but I think we can modify the max_bounce_buffer_size later
potentially directly from machine_set_mem() if the parameter is set.

I'm also interested if anyone has another suggestion for how to solve this
problem more generally.

Thanks,

Jonathan