softmmu: Expand comments describing max_bounce_buffer_size

Clarify how the parameter gets configured and how it is used when
servicing DMA mapping requests targeting indirect memory regions.

Signed-off-by: Mattias Nissler <mnissler@rivosinc.com>
Message-Id: <20240910213512.843130-1-mnissler@rivosinc.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/exec/memory.h

@@ -1104,7 +1104,14 @@ struct AddressSpace {
     QTAILQ_HEAD(, MemoryListener) listeners;
     QTAILQ_ENTRY(AddressSpace) address_spaces_link;
 
-    /* Maximum DMA bounce buffer size used for indirect memory map requests */
+    /*
+     * Maximum DMA bounce buffer size used for indirect memory map requests.
+     * This limits the total size of bounce buffer allocations made for
+     * DMA requests to indirect memory regions within this AddressSpace. DMA
+     * requests that exceed the limit (e.g. due to overly large requested size
+     * or concurrent DMA requests having claimed too much buffer space) will be
+     * rejected and left to the caller to handle.
+     */
     size_t max_bounce_buffer_size;
     /* Total size of bounce buffers currently allocated, atomically accessed */
     size_t bounce_buffer_size;

include/hw/pci/pci_device.h

@@ -168,7 +168,11 @@ struct PCIDevice {
     char *failover_pair_id;
     uint32_t acpi_index;
 
-    /* Maximum DMA bounce buffer size used for indirect memory map requests */
+    /*
+     * Indirect DMA region bounce buffer size as configured for the device. This
+     * is a configuration parameter that is reflected into bus_master_as when
+     * realizing the device.
+     */
     uint32_t max_bounce_buffer_size;
 };