On Tue, Jun 16, 2026 at 03:25:51PM +1000, Gavin Shan wrote:
> All ram device regions were turned to be indirectly accessible by commit
> 4a2e242bbb ("memory: Don't use memcpy for ram_device regions"). This leads
> to guest hang on attempt to build 'cuda-samples' as reported by Julia. The
> guest is started by the following command lines, with GH100 GPU card passed
> from the host.
>
> host$ lspci | grep GH100
> 0009:01:00.0 3D controller: NVIDIA Corporation GH100 [GH200 120GB / 480GB]
> (rev a1)
> host$ /home/sandbox/gavin/qemu.main/build/qemu-system-aarch64 \
> -machine virt,gic-version=host,ras=on,highmem-mmio-size=4T \
> -accel kvm -cpu host -smp cpus=48 -m size=8G \
> -drive file=/home/gavin/sandbox/images/disk.qcow2,if=none,id=d0 \
> -device virtio-blk-pci,id=vb0,bus=pcie.0,drive=d0,num-queues=4 \
> -device vfio-pci-nohotplug,host=0009:01:00.0,bus=pcie.1.0
> :
> guest$ cd cuda-samples/build
> guest$ make -j 20 clean
> guest$ make -j 20
> :
> [ 54%] Linking CUDA executable graphMemoryNodes
> [ 54%] Built target graphMemoryNodes
> <no more output afterwards, guest becomes frozen here>
>
> guest$ qemu-system-aarch64: virtio: bogus descriptor or out of resources
> [ 555.814025] virtio_blk virtio0: [vda] new size: 268435456 512-byte
> logical blocks (137 GB/128 GiB)
>
> When the GPU's driver (NVidia open driver) is loaded on guest bootup,
> the memory blocks residing in the PCI BAR#4 of the GH100 GPU card can
> be presented to the guest through memory hot-add. The page cache can
> then be allocated from the hot added memory blocks when cuda-samples
> is being built. Afterwards, the page cache is sent to QEMU's virtio-blk
> device as part of the DMA request, the bounce buffer has to be used to
> accomodate the request as the corresponding memory region (MemoryRegion)
> is an indirectly accessible ram device region in qemu. However, the max
> bounce bufer size is only 4096 bytes by default and that is exhausted
> quickly, leading to a reset on the virtio-blk device and frozen guest
> eventually.
>
> QEMU
> ====
> virtio_blk_handle_output
> virtio_blk_handle_vq
> virtio_blk_get_request
> virtqueue_pop
> virtqueue_split_pop
> virtqueue_map_desc
> address_space_map
> memory_access_is_direct # Return false
> memory_region_supports_direct_access
>
> (qemu) info mtree
> memory-region: pci_bridge_pci
> 0000000000000000-ffffffffffffffff (prio 0, container): pci_bridge_pci
> 0000042000000000-0000043fffffffff (prio 1, i/o): 0009:01:00.0 base BAR 4
> 0000042000000000-0000043fffffffff (prio 0, i/o): 0009:01:00.0 BAR 4
> 0000042000000000-000004379fffffff (prio 0, ramd): 0009:01:00.0 BAR
> 4 mmaps[0]
>
> This adds qemu_ram_{copy, move}() and replaces {memcpy, memmove}() with
> them in the ram device memory region accessors, similar to what's done
> in commit 4a2e242bbb so that the issue (MMIO access instructions were
> optimized to SSE instructions) covered by that commit is fixed. This
> makes 'ram_device_mem_ops' redundant, paving the way to revert that
> commit to make ram device region directly accessible again in the next
> patch.
>
> Reported-by: Julia Graham <[email protected]>
> Suggested-by: Michael S. Tsirkin <[email protected]>
> Suggested-by: Peter Xu <[email protected]>
> Suggested-by: Richard Henderson <[email protected]>
> Suggested-by: Peter Maydell <[email protected]>
> Signed-off-by: Gavin Shan <[email protected]>
> ---
> v3: 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)
> ---
> hw/remote/vfio-user-obj.c | 4 +-
> include/system/memory.h | 32 ++++++-
> system/physmem.c | 178 +++++++++++++++++++++++++++++++++++++-
> 3 files changed, 207 insertions(+), 7 deletions(-)
>
> diff --git a/hw/remote/vfio-user-obj.c b/hw/remote/vfio-user-obj.c
> index 87fa7b6572..97a6c88780 100644
> --- a/hw/remote/vfio-user-obj.c
> +++ b/hw/remote/vfio-user-obj.c
> @@ -375,9 +375,9 @@ static int vfu_object_mr_rw(MemoryRegion *mr, uint8_t
> *buf, hwaddr offset,
> ram_ptr = memory_region_get_ram_ptr(mr);
>
> if (is_write) {
> - memcpy((ram_ptr + offset), buf, size);
> + qemu_ram_copy(ram_ptr + offset, buf, size);
> } else {
> - memcpy(buf, (ram_ptr + offset), size);
> + qemu_ram_copy(buf, ram_ptr + offset, size);
> }
>
> return 0;
> diff --git a/include/system/memory.h b/include/system/memory.h
> index 1417132f6d..84203c312d 100644
> --- a/include/system/memory.h
> +++ b/include/system/memory.h
> @@ -2897,6 +2897,36 @@ void address_space_register_map_client(AddressSpace
> *as, QEMUBH *bh);
> void address_space_unregister_map_client(AddressSpace *as, QEMUBH *bh);
>
> /* Internal functions, part of the implementation of address_space_read. */
> +
> +/**
> + * qemu_ram_copy: copy data to ramblock
> + *
> + * @dst: destination where the data is copied to
> + * @src: source where the data is copied from
> + * @n: length of data to be copied
> + *
> + *
> + * Copy @n bytes from @src to @dst with the assumption that @src and @dst
> + * do not overlap. Handles special cases such as uncacheable ramblocks
> + * correctly. Use this for accessing ramblock in response to DMA/VCPU IO,
> + * in preference to memcpy().
> + */
> +void qemu_ram_copy(void *dest, const void *src, size_t n);
> +
> +/**
> + * qemu_ram_move: move data to ramblock
> + *
> + * @dst: destination where the data is moved to
> + * @src: source where the data is moved from
> + * @n: length of data to be moved
> + *
> + * Move @n bytes from @src to @dst with the assumption that @src and @dst
> + * can overlap. Handles special cases such as uncacheable ramblocks
> + * correctly. Use this for accessing ramblock in response to DMA/VCPU IO,
> + * in preference to memmove().
> + */
> +void qemu_ram_move(void *dest, const void *src, size_t n);
> +
> MemTxResult address_space_read_full(AddressSpace *as, hwaddr addr,
> MemTxAttrs attrs, void *buf, hwaddr len);
> MemTxResult flatview_read_continue(FlatView *fv, hwaddr addr,
> @@ -2970,7 +3000,7 @@ MemTxResult address_space_read(AddressSpace *as, hwaddr
> addr,
> mr = flatview_translate(fv, addr, &addr1, &l, false, attrs);
> if (len == l && memory_access_is_direct(mr, false, attrs)) {
> ptr = qemu_map_ram_ptr(mr->ram_block, addr1);
> - memcpy(buf, ptr, len);
> + qemu_ram_copy(buf, ptr, len);
> } else {
> result = flatview_read_continue(fv, addr, attrs, buf, len,
> addr1, l, mr);
> diff --git a/system/physmem.c b/system/physmem.c
> index 7bcbf87573..45a17cd580 100644
> --- a/system/physmem.c
> +++ b/system/physmem.c
> @@ -3160,6 +3160,177 @@ void memory_region_flush_rom_device(MemoryRegion *mr,
> hwaddr addr, hwaddr size)
> invalidate_and_set_dirty(mr, addr, size);
> }
>
> +static void qemu_ram_copy_aligned(void *dst, const void *src, size_t n)
> +{
> + switch (n) {
> + case 1:
> + __builtin_memcpy(dst, src, 1);
> + break;
> + case 2:
> + __builtin_memcpy(dst, src, 2);
> + break;
> + case 4:
> + __builtin_memcpy(dst, src, 4);
> + break;
> + case 8:
> + __builtin_memcpy(dst, src, 8);
> + break;
> + default:
> + memcpy(dst, src, n);
> + }
> +}
> +
> +static void qemu_ram_move_aligned(void *dst, const void *src, size_t n)
> +{
> + switch (n) {
> + case 1:
> + __builtin_memmove(dst, src, 1);
> + break;
> + case 2:
> + __builtin_memmove(dst, src, 2);
> + break;
> + case 4:
> + __builtin_memmove(dst, src, 4);
> + break;
> + case 8:
> + __builtin_memmove(dst, src, 8);
> + break;
> + default:
> + memmove(dst, src, n);
> + }
> +}
A weird name for this. You call it for both aligned and unaligned.
maybe this is _unsafe and the "unaligned" below is _safe ?
> +
> +static void qemu_ram_copy_unaligned(void *dst, const void *src,
> + size_t n, size_t max_step)
> +{
> + uintptr_t test, step;
> +
> + /* Aligned maximal step */
> + max_step = pow2floor(max_step);
> +
> + while (n) {
> + test = (uintptr_t)src | (uintptr_t)dst | n | max_step;
> + step = test & -test;
> +
> + switch (step) {
> + case 1:
> + qatomic_set((uint8_t *)dst, qatomic_read((uint8_t *)src));
> + src += 1;
> + dst += 1;
> + n -= 1;
> + break;
> + case 2:
> + qatomic_set((uint16_t *)dst, qatomic_read((uint16_t *)src));
> + src += 2;
> + dst += 2;
> + n -= 2;
> + break;
> + case 4:
> + qatomic_set((uint32_t *)dst, qatomic_read((uint32_t *)src));
> + src += 4;
> + dst += 4;
> + n -= 4;
> + break;
> + case 8:
> + qatomic_set((uint64_t *)dst, qatomic_read((uint64_t *)src));
> + src += 8;
> + dst += 8;
> + n -= 8;
> + break;
> + default:
> + memcpy(dst, src, step);
> + src += step;
> + dst += step;
> + n -= step;
> + }
> + }
> +}
> +
> +static void qemu_ram_backwards_copy_unaligned(void *dst, const void *src,
> + size_t n, size_t max_step)
> +{
> + uintptr_t test, step;
> +
> + /* Aligned maximal step */
> + max_step = pow2floor(max_step);
> +
> + /* End of the blocks */
> + src += n;
> + dst += n;
> +
> + while (n) {
> + test = (uintptr_t)src | (uintptr_t)dst | n | max_step;
> + step = test & -test;
> +
> + switch (step) {
> + case 1:
> + src -= 1;
> + dst -= 1;
> + n -= 1;
> + qatomic_set((uint8_t *)dst, qatomic_read((uint8_t *)src));
> + break;
> + case 2:
> + src -= 2;
> + dst -= 2;
> + n -= 2;
> + qatomic_set((uint16_t *)dst, qatomic_read((uint16_t *)src));
> + break;
> + case 4:
> + src -= 4;
> + dst -= 4;
> + n -= 4;
> + qatomic_set((uint32_t *)dst, qatomic_read((uint32_t *)src));
> + break;
> + case 8:
> + src -= 8;
> + dst -= 8;
> + n -= 8;
> + qatomic_set((uint64_t *)dst, qatomic_read((uint64_t *)src));
> + break;
> + default:
> + src -= step;
> + dst -= step;
> + n -= step;
> + memmove(dst, src, step);
> + }
> + }
> +}
> +
> +/* x86 should work with __builtin_{memcpy, memmove}() for IO access */
> +#if defined(__i386__) || defined(__x86_64__)
> +#define HOST_UNALIGNED_MMIO_OK 1
maybe HOST_UNALIGNED_VECTOR_MMIO_OK ? We also care that vector stores
are safe.
> +#else
> +#define HOST_UNALIGNED_MMIO_OK 0
> +#endif
> +
> +void qemu_ram_copy(void *dst, const void *src, size_t n)
> +{
> + if (dst == src || n == 0) {
> + return;
> + }
> +
> + if (HOST_UNALIGNED_MMIO_OK) {
> + qemu_ram_copy_aligned(dst, src, n);
> + } else {
> + qemu_ram_copy_unaligned(dst, src, n, 8);
> + }
> +}
> +
> +void qemu_ram_move(void *dst, const void *src, size_t n)
> +{
> + if (src == dst || n == 0) {
> + return;
> + }
> +
> + if (HOST_UNALIGNED_MMIO_OK) {
> + qemu_ram_move_aligned(dst, src, n);
> + } else if (dst < src) {
> + qemu_ram_copy_unaligned(dst, src, n, src - dst);
> + } else {
> + qemu_ram_backwards_copy_unaligned(dst, src, n, dst - src);
> + }
> +}
> +
> int memory_access_size(MemoryRegion *mr, unsigned l, hwaddr addr)
> {
> unsigned access_size_max = mr->ops->valid.max_access_size;
> @@ -3272,7 +3443,7 @@ static MemTxResult
> flatview_write_continue_step(MemTxAttrs attrs,
> uint8_t *ram_ptr = qemu_ram_ptr_length(mr->ram_block, mr_addr, l,
> false, true);
>
> - memmove(ram_ptr, buf, *l);
> + qemu_ram_move(ram_ptr, buf, *l);
> invalidate_and_set_dirty(mr, mr_addr, *l);
>
> return MEMTX_OK;
> @@ -3365,7 +3536,7 @@ static MemTxResult
> flatview_read_continue_step(MemTxAttrs attrs, uint8_t *buf,
> uint8_t *ram_ptr = qemu_ram_ptr_length(mr->ram_block, mr_addr, l,
> false, false);
>
> - memcpy(buf, ram_ptr, *l);
> + qemu_ram_copy(buf, ram_ptr, *l);
>
> return MEMTX_OK;
> }
> @@ -3503,8 +3674,7 @@ MemTxResult address_space_write_rom(AddressSpace *as,
> hwaddr addr,
> l = memory_access_size(mr, l, addr1);
> } else {
> /* ROM/RAM case */
> - void *ram_ptr = qemu_map_ram_ptr(mr->ram_block, addr1);
> - memcpy(ram_ptr, buf, l);
> + qemu_ram_copy(qemu_map_ram_ptr(mr->ram_block, addr1), buf, l);
> invalidate_and_set_dirty(mr, addr1, l);
> }
> len -= l;
> --
> 2.54.0
