Thai,

Tao_W_ · ‎04-12-2018

I am testing VMX, and building a Linux kernel module to be as hypervisor.

The Linux kernel module is loaded into a Linux VM, where it is running in VMware workstation.

To test the hypervisor (kernel module), I used the code of bootasm.S http://pages.cs.wisc.edu/~skobov/cs537/P3/xv6/kernel/bootasm.S to build the VM, and loaded it to the hypervisor.

Once the VM is loaded, it can run well, but failed to long jump to start32:
I dumped the VM's GDT, the entries are as follows,
entry 0: 00000000000
entry 1: 00cf9a00,0000ffff
entry 2: 00cf9200,0000ffff
they looked good.
I suspected the instruction of ljmp may not be executed by real mode VM.
The VM's CR0 is 0x60000011, CR4 is 0x2000, RFLAGS is 0x3006.

This problem happened in a nested virtualization environment.

I tested it in a Linux system running in baremetal, it is working well.

I don't know what is missed in my code.

Thanks,

-Tao

Quoc-Thai_L_Intel · ‎04-15-2018

Since you are running under VMware, could this be an issue under the VMware software? You might want to check with them too on this issue. I also have forwarded your question to my peers for any input.

-Thai

Quoc-Thai_L_Intel · ‎04-16-2018

Hello, I got some feedback from my peer to ask you to try something:

Can you try following?

Replace

ljmp $(SEG_KCODE<<3), $start32

with

.byte 0x66, 0xea

.long start32

.word (SEG_KCODE<<3)

Regards,

-Thai

Tao_W_ · ‎04-16-2018

Thai,

Thank you very much for your reply.

I changed the code as you suggested, but it still failed to jump to start32.

Here is the Makefile,

all: bootblock
OBJDUMP=objdump
OBJCOPY=objcopy

CFLAGS = -fno-pic -static -fno-builtin -fno-strict-aliasing -Wall -MD -ggdb -m32 -Werror -fno-omit-frame-pointer
CFLAGS += $(shell $(CC) -fno-stack-protector -E -x c /dev/null >/dev/null 2>&1 && echo -fno-stack-protector)
ASFLAGS = -m32 -gdwarf-2 -Wa,-divide
# FreeBSD ld wants ``elf_i386_fbsd''
LDFLAGS += -m $(shell $(LD) -V | grep elf_i386 2>/dev/null)

bootblock: bootasm.s bootmain.c
$(CC) $(CFLAGS) -fno-pic -nostdinc -I. -c bootasm.S
$(CC) $(CFLAGS) -fno-pic -nostdinc -I. -c bootmain.c
$(LD) $(LDFLAGS) -N -e start -Ttext 0x7C00 -o bootblock.o bootasm.o bootmain.o
$(OBJDUMP) -S bootblock.o > bootblock.asm
$(OBJCOPY) -S -O binary -j .text bootblock.o bootblock.bin

clean:
rm *.o
rm *.bin

the gcc version is

t@ubuntu:~/test/vmxx/kermod/linuxvmxx/toyvmm/vm$ cc -v
Using built-in specs.
COLLECT_GCC=cc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 5.4.0-6ubuntu1~16.04.9' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-5 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.9)

Thanks,

-Tao

Tao_W_ · ‎04-19-2018

Here are more infor for your reference.

The VMCS for the VM

0x0000003F = control_VMX_pin_based
0xA501E1F2 = control_VMX_cpu_based
0x00000082 = control_VMX_proc2_based
0x00000000 = control_exception_bitmap
0x00000000 = control_pagefault_errorcode_mask
0xFFFFFFFF = control_pagefault_errorcode_match
0x00000000 = control_CR3_target_count
0x00036FFB = control_VM_exit_controls
0x000011FB = control_VM_entry_controls
0x00000000 = control_VM_entry_interruption_information
0x00000000 = control_VM_entry_exception_errorcode
0x00000000 = control_VM_entry_instruction_length

0xFFFFFFFFFFFFFFF7 = control_CR0_mask
0xFFFFFFFFFFFFF871 = control_CR4_mask
0x0000000060000010 = control_CR0_shadow
0x0000000000000000 = control_CR4_shadow
0x0000000000000000 = control_CR3_target0
0x00000000B2E98000 = control_CR3_target1
0x0000000000000000 = control_CR3_target2
0x0000000000000000 = control_CR3_target3

The VMX MSRs in the Linux VM ran in VMWare ESXi.

VMX-Capability Model-Specific Registers

00D8100000000001 = IA32_VMX_BASIC_MSR
0000003F00000016 = IA32_VMX_PINBASED_CTLS_MSR
FFF9FFFE0401E172 = IA32_VMX_PROCBASED_CTLS_MSR
003FFFFF00036DFF = IA32_VMX_EXIT_CTLS_MSR
0000F3FF000011FF = IA32_VMX_ENTRY_CTLS_MSR
00000000000401E0 = IA32_VMX_MISC_MSR
0000000080000021 = IA32_VMX_CR0_FIXED0_MSR
00000000FFFFFFFF = IA32_VMX_CR0_FIXED1_MSR
0000000000002000 = IA32_VMX_CR4_FIXED0_MSR
00000000001727FF = IA32_VMX_CR4_FIXED1_MSR
000000000000005A = IA32_VMX_VMCS_ENUM_MSR
000038FE00000000 = IA32_VMX_PROCBASED_CTLS2
00000F0106114141 = IA32_VMX_EPT_VPID_CAP
0000003F00000016 = IA32_VMX_TRUE_PINBASED_CTLS
FFF9FFFE04006172 = IA32_VMX_TRUE_PROCBASED_CTLS
003FFFFF00036DFB = IA32_VMX_TRUE_EXIT_CTLS
0000F3FF000011FB = IA32_VMX_TRUE_ENTRY_CTLS

original_CR0=80050033 PG=1 CD=0 NW=0 AM=1 WP=1 NE=1 ET=1 TS=0 EM=0 MP=1 PE=1
original_CR4=001406E0 VMXE=0 PGE=1 MCE=1 PAE=1 PSE=0 DE=0 TSD=0 PVI=0 VME=0

The guest status just before ljmp

VMX Guest State

CR0=0000000000000031 CR3=0000000000000000 CR4=0000000000002050

RSP=00000000000017FA SYSENTER_ESP=0000000000000000
RIP=000000000000182E SYSENTER_EIP=0000000000000000
DR7=0000000000000400 SYSENTER_CS=00000000 RFLAGS=0000000000000006

ES=0000 [ base=0000000000000000 limit=0000FFFF rights=00000093 ]
CS=0000 [ base=0000000000000000 limit=0000FFFF rights=0000009B ]
SS=0000 [ base=0000000000000000 limit=0000FFFF rights=00000093 ]
DS=0000 [ base=0000000000000000 limit=0000FFFF rights=00000093 ]
FS=0000 [ base=0000000000000000 limit=0000FFFF rights=00000093 ]
GS=0000 [ base=0000000000000000 limit=0000FFFF rights=00000093 ]
LDTR=0000 [ base=0000000000000000 limit=0000FFFF rights=00000082 ]
TR=0000 [ base=0000000000000000 limit=0000FFFF rights=0000008B ]
GDTR [ base=0000000000000000 limit=00000000 ]
IDTR [ base=0000000000000000 limit=0000FFFF ]

EAX=60000011 ECX=00000000 ESI=00000000 ESP=000017FA extints=0
EBX=00000000 EDX=00000000 EDI=00000000 EBP=00000000 nmiints=0

thanks,

-Tao

Quoc-Thai_L_Intel · ‎04-19-2018

Thanks Tao! I have forwarded your additional info. on to my peers.

-Thai

Tao_W_ · ‎04-19-2018

Hi Thai,

Here is the updated guest infor and VMCS for your reference,

VMCS fields.
0x0000003F = control_VMX_pin_based
0xA501E1F2 = control_VMX_cpu_based
0x00000082 = control_VMX_proc2_based
0x00000000 = control_exception_bitmap
0x00000000 = control_pagefault_errorcode_mask
0xFFFFFFFF = control_pagefault_errorcode_match
0x00000000 = control_CR3_target_count
0x00036FFB = control_VM_exit_controls
0x000011FB = control_VM_entry_controls
0x00000000 = control_VM_entry_interruption_information
0x00000000 = control_VM_entry_exception_errorcode
0x00000000 = control_VM_entry_instruction_length

0xFFFFFFFFFFFFFFF7 = control_CR0_mask
0xFFFFFFFFFFFFF871 = control_CR4_mask
0x0000000060000010 = control_CR0_shadow
0x0000000000000000 = control_CR4_shadow
0x0000000000000000 = control_CR3_target0
0x00000000B7934000 = control_CR3_target1
0x0000000000000000 = control_CR3_target2
0x0000000000000000 = control_CR3_target3

Guest state:
CR0=0000000000000031 CR3=0000000000000000 CR4=0000000000002050

RSP=0000000000007BFA SYSENTER_ESP=0000000000000000
RIP=0000000000007C2E SYSENTER_EIP=0000000000000000
DR7=0000000000000400 SYSENTER_CS=00000000 RFLAGS=0000000000000006

ES=0000 [ base=0000000000000000 limit=0000FFFF rights=00000093 ]
CS=0000 [ base=0000000000000000 limit=0000FFFF rights=0000009B ]
SS=0000 [ base=0000000000000000 limit=0000FFFF rights=00000093 ]
DS=0000 [ base=0000000000000000 limit=0000FFFF rights=00000093 ]
FS=0000 [ base=0000000000000000 limit=0000FFFF rights=00000093 ]
GS=0000 [ base=0000000000000000 limit=0000FFFF rights=00000093 ]
LDTR=0000 [ base=0000000000000000 limit=0000FFFF rights=00000082 ]
TR=0000 [ base=0000000000000000 limit=0000FFFF rights=0000008B ]
GDTR [ base=0000000000007C3C limit=00000017 ]
IDTR [ base=0000000000000000 limit=0000FFFF ]

EAX=60000011 ECX=00000000 ESI=00000000 ESP=00007BFA extints=0
EBX=00000000 EDX=00000000 EDI=00000000 EBP=00000000 nmiints=0

The cpuinfo of the Linux running in VMware is below.
processor : 1
vendor_id : GenuineIntel
cpu family : 6
model : 63
model name : Intel(R) Xeon(R) CPU E5-2620 v3 @ 2.40GHz
stepping : 2
microcode : 0x3c
cpu MHz : 2397.291
cache size : 15360 KB
physical id : 2
siblings : 1
core id : 0
cpu cores : 1
apicid : 2
initial apicid : 2
fpu : yes
fpu_exception : yes
cpuid level : 15
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon nopl xtopology nonstop_tsc eagerfpu pni pclmulqdq vmx ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm tpr_shadow vnmi ept vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 invpcid xsaveopt arat
bugs :
bogomips : 4801.89
clflush size : 64
cache_alignment : 64
address sizes : 43 bits physical, 48 bits virtual
power management:

The guest code is as follows,

#define SEG_KCODE 1 // kernel code
#define SEG_KDATA 2 // kernel data+stack
#define SEG_KCPU 3 // kernel per-cpu data
#define SEG_UCODE 4 // user code
#define SEG_UDATA 5 // user data+stack
#define SEG_TSS 6 // this process's task state

#define CR0_PE 0x00000001 // Protection Enable

#define SEG_NULLASM \
.word 0, 0; \
.byte 0, 0, 0, 0

// The 0xC0 means the limit is in 4096-byte units
// and (for executable segments) 32-bit mode.
#define SEG_ASM(type,base,lim) \
.word (((lim) >> 12) & 0xffff), ((base) & 0xffff); \
.byte (((base) >> 16) & 0xff), (0x90 | (type)), \
(0xC0 | (((lim) >> 28) & 0xf)), (((base) >> 24) & 0xff)

#define STA_X 0x8 // Executable segment
#define STA_E 0x4 // Expand down (non-executable segments)
#define STA_C 0x4 // Conforming code segment (executable only)
#define STA_W 0x2 // Writeable (non-executable segments)
#define STA_R 0x2 // Readable (executable segments)
#define STA_A 0x1 // Accessed
# Start the first CPU: switch to 32-bit protected mode, jump into C.

.code16
.global code16, code16_end
code16:
xor %ecx, %ecx
mov %cr3, %eax
mov %eax, %cr3
seta20.1:
inb $0x64,%al # Wait for not busy
testb $0x2,%al
jnz seta20.1

movb $0xd1,%al # 0xd1 -> port 0x64
outb %al,$0x64

seta20.2:
inb $0x64,%al # Wait for not busy
testb $0x2,%al
jnz seta20.2

movb $0xdf,%al # 0xdf -> port 0x60
outb %al,$0x60

wrmsr

lgdt gdtdesc
movl %cr0, %eax
orl $CR0_PE, %eax
movl %eax, %cr0

rdmsr
//PAGEBREAK!
# Complete transition to 32-bit protected mode by using long jmp
# to reload %cs and %eip. The segment descriptors are set up with no
# translation, so that the mapping is still the identity mapping.

.byte 0x66, 0xea
.long start32
.word (SEG_KCODE<<3)

.code32 # Tell assembler to generate 32-bit code now.
start32:
cid:
cpuid
# Bootstrap GDT

.p2align 2 # force 4 byte alignment
gdt:
SEG_NULLASM # NULL seg
SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg
SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg

gdtdesc:

.word (gdtdesc - gdt - 1) # sizeof(gdt) - 1

.long gdt
code16_end:

The Makefile to build it is,

G_CFLAGS = -fno-pic -static -fno-builtin -fno-strict-aliasing -Wall -MD -ggdb -m32 -Werror -fno-omit-frame-pointer
G_CFLAGS += $(shell $(CC) -fno-stack-protector -E -x c /dev/null >/dev/null 2>&1 && echo -fno-stack-protector)

$(CC) $(G_CFLAGS) -fno-pic -nostdinc -I. -c code16.S
$(LD) $(G_LDFLAGS) -N -e start -Ttext 0x7C00 -o bootblock.o code16.o
$(OBJCOPY) -S -O binary -j .text bootblock.o bootblock.bin

Thank you very much for your help.

Failed to enter protected mode in nested virtualization system