Ch 12: QEMU CXL 에뮬레이션 — 노트북에서 CXL 개발

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qemu-system-x86_64 \
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    -machine q35,cxl=on \
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    -m 8G,slots=8,maxmem=32G \
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    -smp 4 \
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    -enable-kvm \
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    ...

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qemu-system-x86_64 \
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    -machine q35,cxl=on \
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    -m 8G,slots=8,maxmem=32G \
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    \
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    -object memory-backend-file,id=cxl-mem0,share=on,\
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            mem-path=./cxl-mem-backing,size=256M \
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    \
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    -device pxb-cxl,bus_nr=12,bus=pcie.0,id=cxl.1 \
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    -device cxl-rp,port=0,bus=cxl.1,id=root_port0,\
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            chassis=0,slot=0 \
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    -device cxl-type3,bus=root_port0,memdev=cxl-mem0,\
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            id=cxl-mem0-dev \
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    \
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    -M cxl-fmw.0.targets.0=cxl.1,cxl-fmw.0.size=512M

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# 커널 6.0+이어야 CXL 서브시스템 동작
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guest$ uname -r
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6.8.0-...
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# CXL 모듈 로딩
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guest$ modprobe cxl_acpi
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guest$ modprobe cxl_pci
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# PCIe로 보임
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guest$ lspci -nn | grep CXL
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0c:00.0 CXL: ... [1af4:0d93]    # virtio vendor + CXL ID
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# CXL sysfs 등록 확인
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guest$ ls /sys/bus/cxl/devices/
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mem0/  decoder0.0/  port0/  root0/
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# 토폴로지
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guest$ cxl list -RT
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[
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  {
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    "root":"root0",
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    "decoders":[
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      {
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        "decoder":"decoder0.0",
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        "size":536870912
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      }
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    ],
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    "endpoints":[
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      {
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        "memdev":"mem0",
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        "ram_size":268435456
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      }
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    ]
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  }
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]
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# Region 생성
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guest$ cxl create-region -d decoder0.0 -t ram -s 256M
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{
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  "region":"region0",
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  "size":268435456,
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  "decoder":"decoder0.0"
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}
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# DAX 모드 또는 system RAM 모드
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guest$ daxctl reconfigure-device dax0.0 -m system-ram
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# numactl로 CXL 노드 확인
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guest$ numactl --hardware
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node 0 size: 8000 MB     # 기본 RAM
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node 1 size: 256 MB      # CXL Type 3 expander

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guest$ acpidump -b
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guest$ iasl -d cedt.dat
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# cedt.dsl 파일 내용
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[001h] Signature              "CEDT"
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[004h] Table Length           0x0000005C
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[008h] Revision               0x01
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[009h] Checksum               0x...
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[Subtable Type: CHBS (CXL Host Bridge Structure)]
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[001h] Subtable Type          0x00
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[003h] UID                    0x0000
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[007h] CXL Version            0x0001
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[00Bh] Base                   0x...
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[013h] Length                 0x...
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[Subtable Type: CFMWS (CXL Fixed Memory Window)]
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[001h] Subtable Type          0x01
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...

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# host에서 cross-compile
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host$ make ARCH=x86_64 CROSS_COMPILE=x86_64-linux-gnu- \
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    M=drivers/cxl/
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# 결과 .ko를 guest로 복사
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host$ scp drivers/cxl/cxl_mock.ko guest:/tmp/
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# guest에서 load·테스트
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guest$ insmod /tmp/cxl_mock.ko
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guest$ dmesg | tail
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guest$ ls /sys/bus/cxl/devices/
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# 수정·반복
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host$ vim drivers/cxl/cxl_mock.c
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host$ make ...

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$ qemu-system-x86_64 -machine pc,cxl=on ...
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qemu-system-x86_64: warning: cxl option requires q35 machine

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$ qemu-system-x86_64 \
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    -object memory-backend-file,id=mem0,\
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            mem-path=/root/cxl-mem,...
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# guest 시작 시 segfault — 권한 거부

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guest$ uname -r
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5.15.0-...
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guest$ modprobe cxl_acpi
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modprobe: FATAL: Module cxl_acpi not found

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-object memory-backend-file,...,size=256M
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-M cxl-fmw.0.size=256M   # 같으면 interleave 영역 없음

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-device cxl-rp,port=0,...,chassis=0,slot=0
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-device cxl-rp,port=1,...,chassis=0,slot=0  # 충돌!