All ram device regions was turned to be indirectly accessible by commit
4a2e242bbb ("memory: Don't use memcpy for ram_device regions"). This leads
to a frozen guest where a NVidia GH100 GPU is passed from host. The memory
in its PCI BAR#4 can be allocated as DMA target buffer. qemu has to take
DMA bounce buffer in address_space_map() to cover the DMA request. However,
the bounce buffer size is 4096 bytes only and it's exhaused very quickly
when the guest has significant disk activities on compiling 'cuda-samples'.
The full log and problem description can be found from PATCH[2/3]'s commit
log.Fix the issue handled in commit 4a2e242bbb by replacing memmove() with newly added qemu_ram_move() where the aligned and small-sized accesses are handled by qatomics, and fall back to memmove() otherwise, for the directly accessible regions. With this, we can revert 4a2e242bbb to make ram device region directly accessible again and bypass the bounce buffer in address_space_map() where the guest hang happens. PATCH[1] replaces memcpy() with memomve() for directly accessible regions PATCH[2] uses qemu_ram_move() for directly accessible regions PATCH[3] makes ram device region directly accessible again Changelog ========= v3 -> v4: * https://lore.kernel.org/qemu-arm/[email protected]/ * New PATCH[1/3] to replace memcpy() with memmove() (Peterx) * Unified qemu_ram_move() for all architectures (Peterx/Michael) v2 -> v3: * https://lore.kernel.org/qemu-arm/[email protected]/ * Documentation for qemu_ram_{copy, move} (Peter/Michael) * Support qemu_ram_move() for overlapped src/dest (Richard) * Use {memcpy, memmove} if step is 16-bytes or more (Michael) * Code improvements (Richard/Michael) v1 -> v2: * https://lore.kernel.org/qemu-arm/[email protected]/ * Rename address_space_{memcpy, memmove}() to qemu_ram_{copy, move}() and move them to physmem.c and memory.h (Philippe) * Use memcpy() and memmove() in qemu_ram_{copy, move}() for the variable length case (Miachel) * Handle unaligned access in qemu_ram_{copy, move}() for all archs except i386 and x86_64 (Richard/Michael) RFCv1 -> v1: * https://lists.nongnu.org/archive/html/qemu-arm/2026-06/msg00307.html * Reworked solution based on suggestions from Peter Xu, Peter Maydell and Michael S. Tsirkin Gavin Shan (3): system/memory: Use memmove() for directly accessible regions system/memory: Use qemu_ram_move() for directly accessible regions system/memory: Make ram device region directly accessible hw/remote/vfio-user-obj.c | 4 ++-- include/system/memory.h | 28 ++++++++++++++++---------- system/memory.c | 41 +-------------------------------------- system/physmem.c | 39 +++++++++++++++++++++++++++++++++++-- system/trace-events | 2 -- 5 files changed, 58 insertions(+), 56 deletions(-) -- 2.55.0
