On 8/8/25 05:50, Xiaoyao Li wrote:
While running the kvm-unit-tests on Intel platforms with "split lock
disable" feature, every test triggers a kernel warning of
x86/split lock detection: #AC: qemu-system-x86_64/373232 took a split_lock
trap at address: 0x1e3
Hack KVM by exiting to QEMU on split lock #AC, we get
KVM: exception 17 exit (error code 0x0)
EAX=00000001 EBX=00000000 ECX=00000014 EDX=0001fb80
ESI=00000000 EDI=000000a8 EBP=00000000 ESP=00006f10
EIP=000001e3 EFL=00010002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0900 00009000 0000ffff 00009300 DPL=0 DS16 [-WA]
CS =c000 000c0000 0000ffff 00009b00 DPL=0 CS16 [-RA]
SS =0000 00000000 0000ffff 00009300 DPL=0 DS16 [-WA]
DS =c000 000c0000 0000ffff 00009300 DPL=0 DS16 [-WA]
FS =0950 00009500 0000ffff 00009300 DPL=0 DS16 [-WA]
GS =06f2 00006f20 0000ffff 00009300 DPL=0 DS16 [-WA]
LDT=0000 00000000 0000ffff 00008200 DPL=0 LDT
TR =0000 00000000 0000ffff 00008b00 DPL=0 TSS32-busy
GDT= 000c02b4 00000027
IDT= 00000000 000003ff
CR0=00000011 CR2=00000000 CR3=00000000 CR4=00000000
DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000
DR3=0000000000000000
DR6=00000000ffff0ff0 DR7=0000000000000400
EFER=0000000000000000
Code=89 16 08 00 65 66 0f 01 16 06 00 66 b8 01 00 00 00 0f 22 c0 <65> 66 ff 2e
00 00 b8 10 00 00 00 8e d0 8e d8 8e c0 8e e0 8e e8 66 b8 08 00 66 ba 10 05 66
And it matches with what disassembled from multiboo_dma.bin:
#objdump -b binary -m i386 -D pc-bios/multiboot_dma.bin
1d1: 08 00 or %al,(%eax)
1d3: 65 66 0f 01 16 lgdtw %gs:(%esi)
1d8: 06 push %es
1d9: 00 66 b8 add %ah,-0x48(%esi)
1dc: 01 00 add %eax,(%eax)
1de: 00 00 add %al,(%eax)
1e0: 0f 22 c0 mov %eax,%cr0
1e3: 65 66 ff 2e ljmpw *%gs:(%esi)
1e7: 00 00 add %al,(%eax)
1e9: b8 10 00 00 00 mov $0x10,%eax
1ee: 8e d0 mov %eax,%ss
1f0: 8e d8 mov %eax,%ds
1f2: 8e c0 mov %eax,%es
1f4: 8e e0 mov %eax,%fs
1f6: 8e e8 mov %eax,%gs
1f8: 66 b8 08 00 mov $0x8,%ax
1fc: 66 ba 10 05 mov $0x510,%dx
We can see that the instruction at 0x1e3 is a far jmp through the GDT.
However, the GDT is not 8 byte aligned, the base is 0xc02b4.
Intel processors follow the LOCK semantics to set the accessed flag of the
segment descriptor when loading a segment descriptor. If the the segment
descriptor crosses two cache line, it causes split lock.
Fix it by aligning the GDT on 8 bytes, so that segment descriptor cannot
span two cache lines.
Cc: qemu-sta...@nongnu.org
Fixes: f16408dfb0e ("Multiboot support")
Signed-off-by: Xiaoyao Li <xiaoyao...@intel.com>
---
pc-bios/multiboot_dma.bin | Bin 1024 -> 1024 bytes
pc-bios/optionrom/multiboot.S | 2 +-
2 files changed, 1 insertion(+), 1 deletion(-)
Reviewed-by: Philippe Mathieu-Daudé <phi...@linaro.org>
