On 7/5/24 16:04, Mattias Nissler wrote:
On Tue, May 7, 2024 at 2:57 PM Philippe Mathieu-Daudé <phi...@linaro.org> wrote:
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 <mniss...@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(-)
/**
* 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.
While switching things over, I was surprised to find that
DEFINE_PROP_SIZE wants a uint64_t field rather than a size_t field.
There is a DEFINE_PROP_SIZE32 variant for uint32_t though. Considering
my options, assuming that we want to use size_t for everything other
than the property:
(1) Make PCIDevice::max_bounce_buffer_size size_t and have the
preprocessor select DEFINE_PROP_SIZE/DEFINE_PROP_SIZE32. This makes
the qdev property type depend on the host. Ugh.
(2) Make PCIDevice::max_bounce_buffer_size uint64_t and clamp if
needed when used. Weird to allow larger values that are then clamped,
although it probably doesn't matter in practice since address space is
limited to 4GB anyways.
(3) Make PCIDevice::max_bounce_buffer_size uint32_t and accept the
limitation that the largest bounce buffer limit is 4GB even on 64-bit
hosts.
#3 seemed most pragmatic, so I'll go with that.
LGTM, thanks for updating.
+ /* Total size of bounce buffers currently allocated, atomically accessed */
+ uint32_t bounce_buffer_size;
Ditto.
