[CXL Volatile MEM]: How to change the CXL persistent memory device to a volatile memory

[CXL Volatile MEM]: How to change the CXL persistent memory device to a volatile memory

[Samarth Saxena]

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Hi All,

I am trying to modify the CXL persistent memory device, given at:
               hw/mem/cxl_type3.c
to a volatile memory device.

I tried the following:

  *   In hw/cxl/cxl-mailbox-utils.c:
     *   changed the IDENTIFY_MEMORY_DEVICE mailbox handler to return persistent capacity as 0, and expose entire memory as volatile.
     *   changed the IDENTIFY_MEMORY_DEVICE mailbox handler to indicate the lba_size as 0 in  the payload.
  *   In hw/mem/cxl_type3.c
     *   Changed function get_lsa_size to return 0.
     *   Changed function get_lsa to return 0.
     *   In the ct3_class_init function, changed the ps->class_id from PCI_CLASS_STORAGE_EXPRESS to PCI_CLASS_MEMORY_RAM
     *   In the ct3_class_init function, set_bit(DEVICE_CLASS_STORGE,dc->categories) was commented
     *   In the setup_cxl_memory function, ensured that the non-volatile flag is false.

Still, when I boot this setup with Kernel 5.14, I see that the device shows up as a persistent memory in /proc/iomem.

What else needs to be changed in the Qemu device and CXL model to support volatile memory?
Is volatile memory Type 3 cxl device not supported in Qemu?

I use the following command:
../build/qemu-system-x86_64 -M q35,accel=kvm,nvdimm=on -m 8192M,slots=4,maxmem=40964M -smp 8,sockets=2,cores=2,threads=2 -hda /lan/dscratch/singhabh/shradha/ubuntu-20.10-64_with_orig_driver_backup.qcow2 -enable-kvm -usbdevice tablet -L $VB_ROOT/etc/vm/common/ -object memory-backend-file,id=cxl-mem1,share=on,mem-path=$PWD/cxl_type_mem,size=512M -object memory-backend-file,id=cxl-lsa,share=on,mem-path=$PWD/cxl_lsa_mem,size=2M -device "pxb-cxl,id=cxl.0,bus=pcie.0,bus_nr=52,uid=0,len-window-base=1,window-base[0]=0x5c0000000,memdev[0]=cxl-mem1" -device cxl-rp,id=rp0,bus=cxl.0,addr=0.0,chassis=0,slot=0 -device cxl-type3,bus=rp0,memdev=cxl-mem1,id=cxl-pmem0,size=256M,lsa=cxl-lsa

Regards,
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Samarth Saxena
Sr Principal Software Engineer
T: +911204308300
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