Re: [PATCH v6 27/43] hw/cxl/host: Add support for CXL Fixed Memory Windows.

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Fri, 4 Mar 2022 15:56:38 +0000
Jonathan Cameron <Jonathan.Cameron@huawei.com> wrote:

> On Wed, 02 Mar 2022 07:55:45 +0100
> Markus Armbruster <armbru@redhat.com> wrote:
> 
> > Jonathan Cameron via <qemu-devel@nongnu.org> writes:
> >   
> > > From: Jonathan Cameron <jonathan.cameron@huawei.com>
> > >
> > > The concept of these is introduced in [1] in terms of the
> > > description the CEDT ACPI table. The principal is more general.
> > > Unlike once traffic hits the CXL root bridges, the host system
> > > memory address routing is implementation defined and effectively
> > > static once observable by standard / generic system software.
> > > Each CXL Fixed Memory Windows (CFMW) is a region of PA space
> > > which has fixed system dependent routing configured so that
> > > accesses can be routed to the CXL devices below a set of target
> > > root bridges. The accesses may be interleaved across multiple
> > > root bridges.
> > >
> > > For QEMU we could have fully specified these regions in terms
> > > of a base PA + size, but as the absolute address does not matter
> > > it is simpler to let individual platforms place the memory regions.
> > >
> > > ExampleS:
> > > -cxl-fixed-memory-window targets=cxl.0,size=128G
> > > -cxl-fixed-memory-window targets=cxl.1,size=128G
> > > -cxl-fixed-memory-window targets=cxl0,targets=cxl.1,size=256G,interleave-granularity=2k
> > >
> > > Specifies
> > > * 2x 128G regions not interleaved across root bridges, one for each of
> > >   the root bridges with ids cxl.0 and cxl.1
> > > * 256G region interleaved across root bridges with ids cxl.0 and cxl.1
> > > with a 2k interleave granularity.
> > >
> > > When system software enumerates the devices below a given root bridge
> > > it can then decide which CFMW to use. If non interleave is desired
> > > (or possible) it can use the appropriate CFMW for the root bridge in
> > > question.  If there are suitable devices to interleave across the
> > > two root bridges then it may use the 3rd CFMS.
> > >
> > > A number of other designs were considered but the following constraints
> > > made it hard to adapt existing QEMU approaches to this particular problem.
> > > 1) The size must be known before a specific architecture / board brings
> > >    up it's PA memory map.  We need to set up an appropriate region.
> > > 2) Using links to the host bridges provides a clean command line interface
> > >    but these links cannot be established until command line devices have
> > >    been added.
> > >
> > > Hence the two step process used here of first establishing the size,
> > > interleave-ways and granularity + caching the ids of the host bridges
> > > and then, once available finding the actual host bridges so they can
> > > be used later to support interleave decoding.
> > >
> > > [1] CXL 2.0 ECN: CEDT CFMWS & QTG DSM (computeexpresslink.org / specifications)
> > >
> > > Signed-off-by: Jonathan Cameron <jonathan.cameron@huawei.com>    
> > 
> > [...]
> >   
> > > diff --git a/hw/cxl/cxl-host.c b/hw/cxl/cxl-host.c
> > > new file mode 100644
> > > index 0000000000..9f303e6d8e
> > > --- /dev/null
> > > +++ b/hw/cxl/cxl-host.c    
> > 
> > [...]
> >   
> > > +QemuOptsList qemu_cxl_fixed_window_opts = {
> > > +    .name = "cxl-fixed-memory-window",
> > > +    .implied_opt_name = "type",
> > > +    .head = QTAILQ_HEAD_INITIALIZER(qemu_cxl_fixed_window_opts.head),
> > > +    .desc = { { 0 } }
> > > +};
> > > +    
> > 
> > [...]
> >   
> > > +static int parse_cxl_fixed_memory_window(void *opaque, QemuOpts *opts,
> > > +                                         Error **errp)
> > > +{
> > > +    CXLFixedMemoryWindowOptions *object = NULL;
> > > +    MachineState *ms = MACHINE(opaque);
> > > +    Error *err = NULL;
> > > +    Visitor *v = opts_visitor_new(opts);
> > > +
> > > +    visit_type_CXLFixedMemoryWindowOptions(v, NULL, &object, errp);
> > > +    visit_free(v);
> > > +    if (!object) {
> > > +        return -1;
> > > +    }
> > > +
> > > +    set_cxl_fixed_memory_window_options(ms, object, &err);
> > > +
> > > +    qapi_free_CXLFixedMemoryWindowOptions(object);
> > > +    if (err) {
> > > +        error_propagate(errp, err);
> > > +        return -1;
> > > +    }
> > > +
> > > +    return 0;
> > > +}
> > > +
> > > +void parse_cxl_fixed_memory_window_opts(MachineState *ms)
> > > +{
> > > +    qemu_opts_foreach(qemu_find_opts("cxl-fixed-memory-window"),
> > > +                      parse_cxl_fixed_memory_window, ms, &error_fatal);
> > > +}    
> > 
> > [...]
> >   
> > > diff --git a/qapi/machine.json b/qapi/machine.json
> > > index 42fc68403d..0998a9128d 100644
> > > --- a/qapi/machine.json
> > > +++ b/qapi/machine.json
> > > @@ -504,6 +504,21 @@
> > >     'dst': 'uint16',
> > >     'val': 'uint8' }}
> > >  
> > > +##
> > > +# @CXLFixedMemoryWindowOptions:
> > > +#
> > > +# Create a CXL Fixed Memory Window (for OptsVisitor)
> > > +#
> > > +# @targets: Target root bridge IDs    
> > 
> > Missing: @size, @targets.
> >   
> > > +#
> > > +# Since X.X //fixme    
> > 
> > Well, "fix me, please".
> >   
> > > +##
> > > +{ 'struct': 'CXLFixedMemoryWindowOptions',
> > > +  'data': {
> > > +      'size': 'size',
> > > +      '*interleave-granularity': 'size',
> > > +      'targets': ['str'] }}
> > > +
> > >  ##
> > >  # @X86CPURegister32:
> > >  #    
> > 
> > [...]
> >   
> > > diff --git a/qemu-options.hx b/qemu-options.hx
> > > index ba3ae6a42a..b4d2cc6f48 100644
> > > --- a/qemu-options.hx
> > > +++ b/qemu-options.hx
> > > @@ -467,6 +467,43 @@ SRST
> > >          -numa hmat-cache,node-id=1,size=10K,level=1,associativity=direct,policy=write-back,line=8
> > >  ERST
> > >  
> > > +DEF("cxl-fixed-memory-window", HAS_ARG, QEMU_OPTION_cxl_fixed_memory_window,
> > > +    "-cxl-fixed-memory-window targets=firsttarget,targets=secondtarget,size=size[,interleave-granularity=granularity]\n",
> > > +    QEMU_ARCH_ALL)
> > > +SRST
> > > +``-cxl-fixed-memory-window targets=firsttarget,targets=secondtarget,size=size[,interleave-granularity=granularity]``
> > > +    Define a CXL Fixed Memory Window (CFMW).
> > > +
> > > +    Described in the CXL 2.0 ECN: CEDT CFMWS & QTG _DSM.
> > > +
> > > +    They are regions of Host Physical Addresses (HPA) on a system which
> > > +    may be interleaved across one or more CXL host bridges.  The system
> > > +    software will assign particular devices into these windows and
> > > +    configure the downstream Host-managed Device Memory (HDM) decoders
> > > +    in root ports, switch ports and devices appropriately to meet the
> > > +    interleave requirements before enabling the memory devices.
> > > +
> > > +    ``targets=firsttarget`` provides the mapping to CXL host bridges
> > > +    which may be identified by the id provied in the -device entry.
> > > +    Multiple entries are needed to specify all the targets when
> > > +    the fixed memory window represents interleaved memory.
> > > +
> > > +    ``size=size`` sets the size of the CFMW. This must be a multiple of
> > > +    256MiB. The region will be aligned to 256MiB but the location is
> > > +    platform and configuration dependent.
> > > +
> > > +    ``interleave-granularity=granularity`` sets the granularity of
> > > +    interleave. Default 256KiB. Only 256KiB, 512KiB, 1024KiB, 2048KiB
> > > +    4096KiB, 8192KiB and 16384KiB granularities supported.
> > > +
> > > +    Example:
> > > +
> > > +    ::
> > > +
> > > +        -cxl-fixed-memory-window -targets=cxl.0,-targets=cxl.1,size=128G,interleave-granularity=512k
> > > +
> > > +ERST
> > > +
> > >  DEF("add-fd", HAS_ARG, QEMU_OPTION_add_fd,
> > >      "-add-fd fd=fd,set=set[,opaque=opaque]\n"
> > >      "                Add 'fd' to fd 'set'\n", QEMU_ARCH_ALL)
> > > diff --git a/softmmu/vl.c b/softmmu/vl.c
> > > index 5e1b35ba48..f83f158fff 100644
> > > --- a/softmmu/vl.c
> > > +++ b/softmmu/vl.c
> > > @@ -91,6 +91,7 @@
> > >  #include "qemu/config-file.h"
> > >  #include "qemu/qemu-options.h"
> > >  #include "qemu/main-loop.h"
> > > +#include "hw/cxl/cxl.h"
> > >  #ifdef CONFIG_VIRTFS
> > >  #include "fsdev/qemu-fsdev.h"
> > >  #endif
> > > @@ -2744,6 +2745,7 @@ void qmp_x_exit_preconfig(Error **errp)
> > >  
> > >      qemu_init_board();
> > >      qemu_create_cli_devices();
> > > +    cxl_fixed_memory_window_link_targets(errp);
> > >      qemu_machine_creation_done();
> > >  
> > >      if (loadvm) {
> > > @@ -2805,6 +2807,7 @@ void qemu_init(int argc, char **argv, char **envp)
> > >      qemu_add_opts(&qemu_msg_opts);
> > >      qemu_add_opts(&qemu_name_opts);
> > >      qemu_add_opts(&qemu_numa_opts);
> > > +    qemu_add_opts(&qemu_cxl_fixed_window_opts);
> > >      qemu_add_opts(&qemu_icount_opts);
> > >      qemu_add_opts(&qemu_semihosting_config_opts);
> > >      qemu_add_opts(&qemu_fw_cfg_opts);
> > > @@ -2927,6 +2930,13 @@ void qemu_init(int argc, char **argv, char **envp)
> > >                      exit(1);
> > >                  }
> > >                  break;
> > > +            case QEMU_OPTION_cxl_fixed_memory_window:
> > > +                opts = qemu_opts_parse_noisily(qemu_find_opts("cxl-fixed-memory-window"),
> > > +                                               optarg, true);
> > > +                if (!opts) {
> > > +                    exit(1);
> > > +                }
> > > +                break;
> > >              case QEMU_OPTION_display:
> > >                  parse_display(optarg);
> > >                  break;
> > > @@ -3764,6 +3774,7 @@ void qemu_init(int argc, char **argv, char **envp)
> > >  
> > >      qemu_resolve_machine_memdev();
> > >      parse_numa_opts(current_machine);
> > > +    parse_cxl_fixed_memory_window_opts(current_machine);
> > >  
> > >      if (vmstate_dump_file) {
> > >          /* dump and exit */    
> > 
> > Have you considered using qobject_input_visitor_new_str() instead of
> > QemuOpts?
> >   
> 
> Umm. No. Why might that be a better approach?

I did some more digging and found some of the discussions on this topic
so seems like a sensible change to make for new API elements.

I have a naive go at this and appears the previous syntax of
targets=cxl.1,targets=cxl.2 etc doesn't work and instead I need to
use targets.0=cxl.1,targets.1=cxl.2

which I seems a little more clunky but I can live with it.

Jonathan

> 
> Thanks,
> 
> Jonathan
> 
>