Re: [PATCH 1/2] system/memory: Use __builtin_mem{cpy, move} in accessors of ram device region

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jun 12, 2026 at 09:03:06PM +1000, Gavin Shan wrote:
> 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 guest hang on compiling 'cuda-samples' as reported by Julia. The guest
> is started by the following command lines, with a GH100 GPU card.
> 
>    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 can be presented to the
> guest through memory hot-add. The page cache can be allocated from the
> hot added memory blocks when cuda-samples is being compiled. 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 a RAM DEVICE region
> and indirectly accessible in qemu. However, the max bounce bufer size
> is only 4096 bytes by default. We're running out of that space quickly.
> 
>   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 replaces mem{cpy, move} with __builtin_mem{cpy, move} in the memory
> accessors to ram device memory region, preparatory work to make ram device
> region directly accessible and bypass the bounce buffer in the DMA path
> in next patch.
> 
> Reported-by: Julia Graham <jugraham@redhat.com>
> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> Suggested-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>  hw/remote/vfio-user-obj.c |  4 ++--
>  include/system/memory.h   | 42 ++++++++++++++++++++++++++++++++++++++-
>  system/physmem.c          |  8 ++++----
>  3 files changed, 47 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/remote/vfio-user-obj.c b/hw/remote/vfio-user-obj.c
> index 87fa7b6572..fe6f661fe2 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);
> +            address_space_memcpy(ram_ptr + offset, buf, size);
>          } else {
> -            memcpy(buf, (ram_ptr + offset), size);
> +            address_space_memcpy(buf, ram_ptr + offset, size);
>          }
>  
>          return 0;
> diff --git a/include/system/memory.h b/include/system/memory.h
> index 1417132f6d..6bb2e13eea 100644
> --- a/include/system/memory.h
> +++ b/include/system/memory.h
> @@ -2938,6 +2938,46 @@ static inline bool memory_access_is_direct(const MemoryRegion *mr,
>      return true;
>  }
>  
> +static inline void address_space_memcpy(void *dest, const void *src, size_t n)
> +{
> +    switch (n) {
> +    case 1:
> +        __builtin_memcpy(dest, src, 1);
> +        break;
> +    case 2:
> +        __builtin_memcpy(dest, src, 2);
> +        break;
> +    case 4:
> +        __builtin_memcpy(dest, src, 4);
> +        break;
> +    case 8:
> +        __builtin_memcpy(dest, src, 8);
> +        break;
> +    default:
> +        __builtin_memcpy(dest, src, n);
> +    }
> +}
> +
> +static inline void address_space_memmove(void *dest, const void *src, size_t n)
> +{
> +    switch (n) {
> +    case 1:
> +        __builtin_memmove(dest, src, 1);
> +        break;
> +    case 2:
> +        __builtin_memmove(dest, src, 2);
> +        break;
> +    case 4:
> +        __builtin_memmove(dest, src, 4);
> +        break;
> +    case 8:
> +        __builtin_memmove(dest, src, 8);
> +        break;
> +    default:
> +        __builtin_memmove(dest, src, n);
> +    }
> +}
> +
>  /**
>   * address_space_read: read from an address space.
>   *


The variable length probably should use the regular memcpy/memmove -
no reason to bypass fortification for these.


> @@ -2970,7 +3010,7 @@ 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);
> +                __builtin_memcpy(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..5f46a9d676 100644
> --- a/system/physmem.c
> +++ b/system/physmem.c
> @@ -3272,7 +3272,7 @@ 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);
> +        address_space_memmove(ram_ptr, buf, *l);
>          invalidate_and_set_dirty(mr, mr_addr, *l);
>  
>          return MEMTX_OK;
> @@ -3365,7 +3365,7 @@ 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);
> +        address_space_memcpy(buf, ram_ptr, *l);
>  
>          return MEMTX_OK;
>      }
> @@ -3503,8 +3503,8 @@ 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);
> +            address_space_memcpy(qemu_map_ram_ptr(mr->ram_block, addr1),
> +                                 buf, l);
>              invalidate_and_set_dirty(mr, addr1, l);
>          }
>          len -= l;
> -- 
> 2.54.0