Re: [PATCH v3 2/5] softmmu: Support concurrent bounce buffers

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, 15 Sept 2023 at 05:55, Mattias Nissler <mnissler@rivosinc.com> wrote:
>
> On Thu, Sep 14, 2023 at 8:49 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > On Thu, Sep 07, 2023 at 06:04:07AM -0700, 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 ++
> > >  softmmu/memory.c            |  3 +-
> > >  softmmu/physmem.c           | 94 +++++++++++++++++++++++++------------
> > >  5 files changed, 80 insertions(+), 42 deletions(-)
> > >
> > > diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> > > index 881d774fb6..8c4541b394 100644
> > > --- a/hw/pci/pci.c
> > > +++ b/hw/pci/pci.c
> > > @@ -85,6 +85,8 @@ static Property pci_props[] = {
> > >                      QEMU_PCIE_ERR_UNC_MASK_BITNR, true),
> > >      DEFINE_PROP_BIT("x-pcie-ari-nextfn-1", PCIDevice, cap_present,
> > >                      QEMU_PCIE_ARI_NEXTFN_1_BITNR, false),
> > > +    DEFINE_PROP_SIZE("x-max-bounce-buffer-size", PCIDevice,
> > > +                     max_bounce_buffer_size, 4096),
> > >      DEFINE_PROP_END_OF_LIST()
> > >  };
> > >
> > > @@ -1208,6 +1210,8 @@ static PCIDevice *do_pci_register_device(PCIDevice *pci_dev,
> > >                         "bus master container", UINT64_MAX);
> > >      address_space_init(&pci_dev->bus_master_as,
> > >                         &pci_dev->bus_master_container_region, pci_dev->name);
> > > +    pci_dev->bus_master_as.max_bounce_buffer_size =
> > > +        pci_dev->max_bounce_buffer_size;
> > >
> > >      if (phase_check(PHASE_MACHINE_READY)) {
> > >          pci_init_bus_master(pci_dev);
> > > @@ -2664,6 +2668,10 @@ static void pci_device_class_init(ObjectClass *klass, void *data)
> > >      k->unrealize = pci_qdev_unrealize;
> > >      k->bus_type = TYPE_PCI_BUS;
> > >      device_class_set_props(k, pci_props);
> > > +    object_class_property_set_description(
> > > +        klass, "x-max-bounce-buffer-size",
> > > +        "Maximum buffer size allocated for bounce buffers used for mapped "
> > > +        "access to indirect DMA memory");
> > >  }
> > >
> > >  static void pci_device_class_base_init(ObjectClass *klass, void *data)
> > > diff --git a/include/exec/memory.h b/include/exec/memory.h
> > > index 7d68936157..5577542b5e 100644
> > > --- a/include/exec/memory.h
> > > +++ b/include/exec/memory.h
> > > @@ -1081,14 +1081,6 @@ typedef struct AddressSpaceMapClient {
> > >      QLIST_ENTRY(AddressSpaceMapClient) link;
> > >  } AddressSpaceMapClient;
> > >
> > > -typedef struct {
> > > -    MemoryRegion *mr;
> > > -    void *buffer;
> > > -    hwaddr addr;
> > > -    hwaddr len;
> > > -    bool in_use;
> > > -} BounceBuffer;
> > > -
> > >  /**
> > >   * struct AddressSpace: describes a mapping of addresses to #MemoryRegion objects
> > >   */
> > > @@ -1106,8 +1098,10 @@ 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 */
> > > +    uint64_t max_bounce_buffer_size;
> > > +    /* Total size of bounce buffers currently allocated, atomically accessed */
> > > +    uint64_t bounce_buffer_size;
> > >      /* 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..f4027c5379 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 */
> > > +    uint64_t max_bounce_buffer_size;
> > >  };
> > >
> > >  static inline int pci_intx(PCIDevice *pci_dev)
> > > diff --git a/softmmu/memory.c b/softmmu/memory.c
> > > index 5c9622c3d6..e02799359c 100644
> > > --- a/softmmu/memory.c
> > > +++ b/softmmu/memory.c
> > > @@ -3105,7 +3105,8 @@ void address_space_init(AddressSpace *as, MemoryRegion *root, const char *name)
> > >      as->ioeventfds = NULL;
> > >      QTAILQ_INIT(&as->listeners);
> > >      QTAILQ_INSERT_TAIL(&address_spaces, as, address_spaces_link);
> > > -    as->bounce.in_use = false;
> > > +    as->max_bounce_buffer_size = 4096;
> > > +    as->bounce_buffer_size = 0;
> > >      qemu_mutex_init(&as->map_client_list_lock);
> > >      QLIST_INIT(&as->map_client_list);
> > >      as->name = g_strdup(name ? name : "anonymous");
> > > diff --git a/softmmu/physmem.c b/softmmu/physmem.c
> > > index f40cc564b8..e3d1cf5fba 100644
> > > --- a/softmmu/physmem.c
> > > +++ b/softmmu/physmem.c
> > > @@ -2926,6 +2926,20 @@ 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;
> > > +    size_t len;
> > > +    uint8_t buffer[];
> > > +} BounceBuffer;
> > > +
> > >  static void
> > >  address_space_unregister_map_client_do(AddressSpaceMapClient *client)
> > >  {
> > > @@ -2953,7 +2967,7 @@ void address_space_register_map_client(AddressSpace *as, QEMUBH *bh)
> > >      QLIST_INSERT_HEAD(&as->map_client_list, client, link);
> > >      /* 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);
> > > @@ -3081,31 +3095,36 @@ void *address_space_map(AddressSpace *as,
> > >      RCU_READ_LOCK_GUARD();
> > >      fv = address_space_to_flatview(as);
> > >      mr = flatview_translate(fv, addr, &xlat, &l, is_write, attrs);
> > > +    memory_region_ref(mr);
> > >
> > >      if (!memory_access_is_direct(mr, is_write)) {
> > > -        if (qatomic_xchg(&as->bounce.in_use, true)) {
> > > +        size_t size = qatomic_add_fetch(&as->bounce_buffer_size, l);
> > > +        if (size > as->max_bounce_buffer_size) {
> > > +            size_t excess = size - as->max_bounce_buffer_size;
> > > +            l -= excess;
> > > +            qatomic_sub(&as->bounce_buffer_size, excess);
> >
> > I think two threads can race here. as->bounce_buffer_size will be
> > corrupted (smaller than it should be) and l will be wrong as well. A
> > cmpxchg loop would solve the race.
>
> Ah, thanks for pointing this out. I think the case you have in mind is this:
> 1. Thread A bumps the size to be larger than the max
> 2. Thread B bumps the size further
> 3. Thread B now computes an incorrect excess and sutracts more than it added.

Yes, that's the case.

> I should be good if I ensure that each thread will only subtract what
> it has previously added to enforce the invariant that additions and
> subtractions for each map/unmap pair cancel each other out. Let me
> know if there's a reason to still prefer the cmpxchg loop.

Cool, it would be nice to avoid the cmpxchg loop.

Stefan