On Wed, Sep 11, 2024 at 12:24 PM Michael S. Tsirkin <m...@redhat.com> wrote:
>
> On Tue, Sep 10, 2024 at 11:36:08PM +0200, Mattias Nissler wrote:
> > On Tue, Sep 10, 2024 at 6:40 PM Michael S. Tsirkin <m...@redhat.com> wrote:
> > >
> > > On Tue, Sep 10, 2024 at 06:10:50PM +0200, Mattias Nissler wrote:
> > > > On Tue, Sep 10, 2024 at 5:44 PM Peter Maydell
> > > > <peter.mayd...@linaro.org> wrote:
> > > > >
> > > > > 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.
> > > >
> > > > One is one the pci device, the other is on the address space. The
> > > > former can be set via a command line parameter, and that value is used
> > > > to initialize the field on the address space, which is then consulted
> > > > when allocating bounce buffers.
> > > >
> > > > I'm not sure which aspect of this is unclear and/or deserves
> > > > additional commenting - let me know and I'll be happy to send a patch.
> > >
> > > I'd document what does each field do.
> >
> > I have just sent a patch to expand the comments, let's discuss details
> > there.
> >
> > >
> > > > > >
> > > > > > 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.
> >
> > All I needed was a practical way to allow the bounce buffer size limit
> > to be adjusted in the somewhat exotic situations where we're making
> > DMA requests to indirect memory regions (in my case it is a qemu
> > vfio-user server accessed by a client that can't or doesn't want to
> > provide direct memory-mapped access to its RAM). There was some
> > discussion about the nature of the parameter when I first proposed the
> > patch, see
> > https://lore.kernel.org/qemu-devel/20230823092905.2259418-2-mniss...@rivosinc.com/
> > - an x-prefixed experimental command-line parameter was suggested
> > there as a practical way to allow this without qemu committing to
> > supporting this forever. For the unlikely case that this parameter
> > proves popular, it can still be added to a stable API (or
> > alternatively we could discuss whether a large-enough limit is
> > feasible after all, or even consider DMA API changes to obviate the
> > need for bounce buffering).
>
>
> Yes but how happy will you be if we rename the parameter in the
> future? All your scripts will break.
It's not that I'm running random qemu versions in production, in fact
I'm using this for semi-automated testing of hardware designs. We'd
find out when upgrading our qemu, and adjust. In fact, if you come up
with a better way to handle this bounce buffering kludge, I'd be
willing to not only adjust, but even help implement.
>
> > > > >
> > > > > Note that this patch is already upstream as commit 637b0aa13.
> > > > >
> > > > > thanks
> > > > > -- PMM
> > >
> > > Maybe you can answer this?
> > >
>
