On Tue, 10 Sept 2024 at 15:53, Michael S. Tsirkin <m...@redhat.com> wrote:
>
> On Mon, Aug 19, 2024 at 06:54:54AM -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 <mniss...@rivosinc.com>
> > Reviewed-by: Philippe Mathieu-Daudé <phi...@linaro.org>
> > Acked-by: Peter Xu <pet...@redhat.com>
> > ---
> > This patch is split out from my "Support message-based DMA in vfio-user
> > server"
> > series. With the series having been partially applied, I'm splitting this
> > one
> > out as the only remaining patch to system emulation code in the hope to
> > simplify getting it landed. The code has previously been reviewed by Stefan
> > Hajnoczi and Peter Xu. This latest version includes changes to switch the
> > bounce buffer size bookkeeping to `size_t` as requested and LGTM'd by Phil
> > in
> > v9.
> > ---
> > 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/hw/pci/pci.c b/hw/pci/pci.c
> > index fab86d0567..d2caf3ee8b 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_SIZE32("x-max-bounce-buffer-size", PCIDevice,
> > + max_bounce_buffer_size,
> > DEFAULT_MAX_BOUNCE_BUFFER_SIZE),
> > DEFINE_PROP_END_OF_LIST()
> > };
> >
>
> I'm a bit puzzled by now there being two fields named
> max_bounce_buffer_size, one directly controllable by
> a property.
>
> Pls add code comments explaining how they are related.
>
>
> Also, what is the point of adding a property without
> making it part of an API? No one will be able to rely on
> it working.
Note that this patch is already upstream as commit 637b0aa13.
thanks
-- PMM
