Re: [PATCH v9 2/5] softmmu: Support concurrent bounce buffers

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 7/5/24 11:42, Mattias Nissler wrote:
> When DMA memory can't be directly accessed, as is the case when
> running the device model in a separate process without shareable DMA
> file descriptors, bounce buffering is used.
> 
> It is not uncommon for device models to request mapping of several DMA
> regions at the same time. Examples include:
>   * net devices, e.g. when transmitting a packet that is split across
>     several TX descriptors (observed with igb)
>   * USB host controllers, when handling a packet with multiple data TRBs
>     (observed with xhci)
> 
> Previously, qemu only provided a single bounce buffer per AddressSpace
> and would fail DMA map requests while the buffer was already in use. In
> turn, this would cause DMA failures that ultimately manifest as hardware
> errors from the guest perspective.
> 
> This change allocates DMA bounce buffers dynamically instead of
> supporting only a single buffer. Thus, multiple DMA mappings work
> correctly also when RAM can't be mmap()-ed.
> 
> The total bounce buffer allocation size is limited individually for each
> AddressSpace. The default limit is 4096 bytes, matching the previous
> maximum buffer size. A new x-max-bounce-buffer-size parameter is
> provided to configure the limit for PCI devices.
> 
> Signed-off-by: Mattias Nissler <mnissler@rivosinc.com>
> ---
>   hw/pci/pci.c                |  8 ++++
>   include/exec/memory.h       | 14 +++----
>   include/hw/pci/pci_device.h |  3 ++
>   system/memory.c             |  5 ++-
>   system/physmem.c            | 82 ++++++++++++++++++++++++++-----------
>   5 files changed, 76 insertions(+), 36 deletions(-)


> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index d417d7f363..2ea1e99da2 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -1117,13 +1117,7 @@ typedef struct AddressSpaceMapClient {
>       QLIST_ENTRY(AddressSpaceMapClient) link;
>   } AddressSpaceMapClient;
>   
> -typedef struct {
> -    MemoryRegion *mr;
> -    void *buffer;
> -    hwaddr addr;
> -    hwaddr len;
> -    bool in_use;
> -} BounceBuffer;
> +#define DEFAULT_MAX_BOUNCE_BUFFER_SIZE (4096)
>   
>   /**
>    * struct AddressSpace: describes a mapping of addresses to #MemoryRegion objects
> @@ -1143,8 +1137,10 @@ struct AddressSpace {
>       QTAILQ_HEAD(, MemoryListener) listeners;
>       QTAILQ_ENTRY(AddressSpace) address_spaces_link;
>   
> -    /* Bounce buffer to use for this address space. */
> -    BounceBuffer bounce;
> +    /* Maximum DMA bounce buffer size used for indirect memory map requests */
> +    uint32_t max_bounce_buffer_size;

Alternatively size_t.

> +    /* Total size of bounce buffers currently allocated, atomically accessed */
> +    uint32_t bounce_buffer_size;

Ditto.

>       /* List of callbacks to invoke when buffers free up */
>       QemuMutex map_client_list_lock;
>       QLIST_HEAD(, AddressSpaceMapClient) map_client_list;
> diff --git a/include/hw/pci/pci_device.h b/include/hw/pci/pci_device.h
> index d3dd0f64b2..253b48a688 100644
> --- a/include/hw/pci/pci_device.h
> +++ b/include/hw/pci/pci_device.h
> @@ -160,6 +160,9 @@ struct PCIDevice {
>       /* ID of standby device in net_failover pair */
>       char *failover_pair_id;
>       uint32_t acpi_index;
> +
> +    /* Maximum DMA bounce buffer size used for indirect memory map requests */
> +    uint32_t max_bounce_buffer_size;

Ditto.

>   };


> diff --git a/system/physmem.c b/system/physmem.c
> index 632da6508a..cd61758da0 100644
> --- a/system/physmem.c
> +++ b/system/physmem.c
> @@ -3046,6 +3046,20 @@ void cpu_flush_icache_range(hwaddr start, hwaddr len)
>                                        NULL, len, FLUSH_CACHE);
>   }
>   
> +/*
> + * A magic value stored in the first 8 bytes of the bounce buffer struct. Used
> + * to detect illegal pointers passed to address_space_unmap.
> + */
> +#define BOUNCE_BUFFER_MAGIC 0xb4017ceb4ffe12ed
> +
> +typedef struct {
> +    uint64_t magic;
> +    MemoryRegion *mr;
> +    hwaddr addr;
> +    uint32_t len;
> +    uint8_t buffer[];
> +} BounceBuffer;

Eh, you moved it back here. Never mind.

> +
>   static void
>   address_space_unregister_map_client_do(AddressSpaceMapClient *client)
>   {
> @@ -3071,9 +3085,9 @@ void address_space_register_map_client(AddressSpace *as, QEMUBH *bh)
>       qemu_mutex_lock(&as->map_client_list_lock);
>       client->bh = bh;
>       QLIST_INSERT_HEAD(&as->map_client_list, client, link);
> -    /* Write map_client_list before reading in_use.  */
> +    /* Write map_client_list before reading bounce_buffer_size. */
>       smp_mb();
> -    if (!qatomic_read(&as->bounce.in_use)) {
> +    if (qatomic_read(&as->bounce_buffer_size) < as->max_bounce_buffer_size) {
>           address_space_notify_map_clients_locked(as);
>       }
>       qemu_mutex_unlock(&as->map_client_list_lock);
> @@ -3203,28 +3217,40 @@ void *address_space_map(AddressSpace *as,
>       mr = flatview_translate(fv, addr, &xlat, &l, is_write, attrs);
>   
>       if (!memory_access_is_direct(mr, is_write)) {
> -        if (qatomic_xchg(&as->bounce.in_use, true)) {
> +        uint32_t used = qatomic_read(&as->bounce_buffer_size);

Nitpicking again, size_t seems clearer. Otherwise LGTM.