RE: [PATCH] hw/arm: add static NVDIMMs in device tree

[Shameerali Kolothum Thodi via <qemu-arm@nongnu.org>]

> -----Original Message-----
> From: Jonathan Cameron <jonathan.cameron@huawei.com>
> Sent: Thursday, July 31, 2025 11:01 AM
> To: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
> Cc: qemu-devel@nongnu.org; Peter Maydell <peter.maydell@linaro.org>;
> qemu-arm@nongnu.org; Gustavo Romero <gustavo.romero@linaro.org>;
> Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
> Subject: Re: [PATCH] hw/arm: add static NVDIMMs in device tree
> 
> On Wed, 30 Jul 2025 15:21:41 +0300
> Manos Pitsidianakis <manos.pitsidianakis@linaro.org> wrote:
> 
> > NVDIMM is used for fast rootfs with EROFS, for example by kata
> > containers. To allow booting with static NVDIMM memory, add them to
> > the device tree in arm virt machine.
> >
> > This allows users to boot directly with nvdimm memory devices without
> > having to rely on ACPI and hotplug.
> >
> > Verified to work with command invocation:
> >
> > ./qemu-system-aarch64 \
> >   -M virt,nvdimm=on \
> >   -cpu cortex-a57 \
> >   -m 4G,slots=2,maxmem=8G \
> >   -object memory-backend-file,id=mem1,share=on,mem-
> path=/tmp/nvdimm,size=4G,readonly=off \
> >   -device nvdimm,id=nvdimm1,memdev=mem1,unarmed=off \
> >   -drive file=./debian-12-nocloud-arm64-commited.qcow2,format=qcow2 \
> >   -kernel ./vmlinuz-6.1.0-13-arm64 \
> >   -append "root=/dev/vda1 console=ttyAMA0,115200 acpi=off"
> >   -initrd ./initrd.img-6.1.0-13-arm64 \
> >   -nographic \
> >   -serial mon:stdio
> >
> > Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
> 
> +CC shameer who might be able to remember how the nvdimm stuff works
> in
> +ACPI better
> than I can.  I think this is fine but more eyes would be good.

The cold plug DT support was part of the initial NVDIMM series,
https://lore.kernel.org/qemu-devel/20191004155302.4632-5-shameerali.kolothum.thodi@huawei.com/

But I can't remember the reason for dropping it, other than the comment from
Igor, that why we should do it for NVDIMM but not PC-DIMM.
https://lore.kernel.org/qemu-devel/20191111154627.63fc061b@redhat.com/

So, I guess there was not a strong use case for that at that time.

The PC-DIMM DT cold plug was dropped due to the issues/obstacles mentioned here,
https://lore.kernel.org/qemu-devel/5FC3163CFD30C246ABAA99954A238FA83F1B6A66@lhreml524-mbs.china.huawei.com/

+CC: Igor and Eric.

Thanks,
Shameer

> > ---
> >  hw/arm/boot.c | 39 +++++++++++++++++++++++++++++++++++++++
> >  hw/arm/virt.c |  8 +++++---
> >  2 files changed, 44 insertions(+), 3 deletions(-)
> >
> > diff --git a/hw/arm/boot.c b/hw/arm/boot.c index
> >
> d391cd01bb1b92ff213e69b84e5a69413b36c4f8..a0c1bcdf946ca98bb5da63f1
> a518
> > 018eb578dd81 100644
> > --- a/hw/arm/boot.c
> > +++ b/hw/arm/boot.c
> > @@ -25,6 +25,7 @@
> >  #include "hw/boards.h"
> >  #include "system/reset.h"
> >  #include "hw/loader.h"
> > +#include "hw/mem/memory-device.h"
> >  #include "elf.h"
> >  #include "system/device_tree.h"
> >  #include "qemu/config-file.h"
> > @@ -515,6 +516,26 @@ static void fdt_add_psci_node(void *fdt,
> ARMCPU *armcpu)
> >      qemu_fdt_setprop_cell(fdt, "/psci", "migrate", migrate_fn);  }
> >
> > +static int fdt_add_pmem_node(void *fdt, uint32_t acells, uint32_t scells,
> > +                             int64_t mem_base, int64_t size, int64_t
> > +node) {
> > +    int ret;
> > +
> > +    g_autofree char *nodename = g_strdup_printf("/pmem@%" PRIx64,
> > + mem_base);
> > +
> > +    qemu_fdt_add_subnode(fdt, nodename);
> > +    qemu_fdt_setprop_string(fdt, nodename, "compatible", "pmem-
> region");
> > +    ret = qemu_fdt_setprop_sized_cells(fdt, nodename, "reg", acells,
> > +                                       mem_base, scells, size);
> 
> I'd burn some lines to avoid a comment covering unrelated ret handling
> 
> 	if (ret)
> 		return ret;
> 
> 	if (node >= 0) {
> 		return qem_fdt_setprop_cell()
> 	}
> 
> 	return 0;
> 
> > +    /* only set the NUMA ID if it is specified */
> > +    if (!ret && node >= 0) {
> > +        ret = qemu_fdt_setprop_cell(fdt, nodename, "numa-node-id",
> > +                                    node);
> > +    }
> > +
> > +    return ret;
> > +}
> > +
> >  int arm_load_dtb(hwaddr addr, const struct arm_boot_info *binfo,
> >                   hwaddr addr_limit, AddressSpace *as, MachineState *ms,
> >                   ARMCPU *cpu)
> > @@ -525,6 +546,7 @@ int arm_load_dtb(hwaddr addr, const struct
> arm_boot_info *binfo,
> >      unsigned int i;
> >      hwaddr mem_base, mem_len;
> >      char **node_path;
> > +    g_autofree MemoryDeviceInfoList *md_list = NULL;
> >      Error *err = NULL;
> >
> >      if (binfo->dtb_filename) {
> > @@ -628,6 +650,23 @@ int arm_load_dtb(hwaddr addr, const struct
> arm_boot_info *binfo,
> >          }
> >      }
> >
> > +    md_list = qmp_memory_device_list();
> > +    for (MemoryDeviceInfoList *m = md_list; m != NULL; m = m->next) {
> > +        MemoryDeviceInfo *mi = m->value;
> > +
> > +        if (mi->type == MEMORY_DEVICE_INFO_KIND_NVDIMM) {
> > +            PCDIMMDeviceInfo *di = mi->u.nvdimm.data;
> > +
> > +            rc = fdt_add_pmem_node(fdt, acells, scells,
> > +                                   di->addr, di->size, di->node);
> > +            if (rc < 0) {
> > +                fprintf(stderr, "couldn't add NVDIMM /pmem@%"PRIx64"
> node\n",
> > +                        di->addr);
> > +                goto fail;
> > +            }
> > +        }
> > +    }
> > +
> >      rc = fdt_path_offset(fdt, "/chosen");
> >      if (rc < 0) {
> >          qemu_fdt_add_subnode(fdt, "/chosen"); diff --git
> > a/hw/arm/virt.c b/hw/arm/virt.c index
> >
> ef6be3660f5fb38da84235c32dc2d13a5c61889c..910f5bb5f66ee217a9140f912
> 880
> > 4a5b9f69b5b6 100644
> > --- a/hw/arm/virt.c
> > +++ b/hw/arm/virt.c
> > @@ -2917,7 +2917,7 @@ static void
> virt_memory_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
> >      const MachineState *ms = MACHINE(hotplug_dev);
> >      const bool is_nvdimm = object_dynamic_cast(OBJECT(dev),
> > TYPE_NVDIMM);
> >
> > -    if (!vms->acpi_dev) {
> > +    if (!vms->acpi_dev && !(is_nvdimm && !dev->hotplugged)) {
> >          error_setg(errp,
> >                     "memory hotplug is not enabled: missing acpi-ged device");
> >          return;
> > @@ -2949,8 +2949,10 @@ static void virt_memory_plug(HotplugHandler
> *hotplug_dev,
> >          nvdimm_plug(ms->nvdimms_state);
> >      }
> >
> > -    hotplug_handler_plug(HOTPLUG_HANDLER(vms->acpi_dev),
> > -                         dev, &error_abort);
> > +    if (vms->acpi_dev) {
> > +        hotplug_handler_plug(HOTPLUG_HANDLER(vms->acpi_dev),
> > +                             dev, &error_abort);
> > +    }
> >  }
> >
> >  static void virt_machine_device_pre_plug_cb(HotplugHandler
> > *hotplug_dev,
> >
> > ---
> > base-commit: 9b80226ece693197af8a981b424391b68b5bc38e
> > change-id: 20250730-nvdimm_arm64_virt-931a764bbe0c
> >
> > --
> > γαῖα πυρί μιχθήτω
> >
> >