Hi,
On x86, when executing a VM in single-step mode (from kvmtool), one
instruction may be skipped after some instructions. Following is a way
to reproduce.
Examples of instruction that triggers the behavior (intel syntax):
- out (but not in),
- rdmsr, wrmsr
- mov cr0,ax (but not mov ax, cr0)
- wbinvd
- ...
The skipped instruction is executed as this can be checked with the
details of the single-step information but fail to appear when running
kvmtool --debug-single-step option.
This is a little annoyance but I wonder if it does not make surface a
deeper issue in kvm debugger support.
FF
cat /etc/lsb-release
#DISTRIB_ID=Ubuntu
#DISTRIB_RELEASE=16.10
#DISTRIB_CODENAME=yakkety
#DISTRIB_DESCRIPTION="Ubuntu 16.10"
modinfo kvm
#filename: /lib/modules/4.8.0-34-generic/kernel/arch/x86/kvm/kvm.ko
#license: GPL
#author: Qumranet
#srcversion: 07F4273C7963D92A67AC6C8
#depends: irqbypass
#intree: Y
#vermagic: 4.8.0-34-generic SMP mod_unload modversions
uname -a
#Linux XXXXXXXX 4.8.0-34-generic #36-Ubuntu SMP Wed Dec 21 17:24:18
UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
cat <<EOF > nop.S
[SECTION .code]
[BITS 16]
start:
mov ax,1
mov bx,ax
wbinvd
mov ax,2
wbinvd
mov ax,3
hlt
EOF
nasm nop.S -o nop.bin
#ndisasm -b 16 nop.bin
# gives
#00000000 B80100 mov ax,0x1
#00000003 89C3 mov bx,ax
#00000005 0F09 wbinvd
#00000007 B80200 mov ax,0x2
#0000000A 0F09 wbinvd
#0000000C B80300 mov ax,0x3
#0000000F F4 hlt
kvmtool/lkvm run --cpus 1 --mem 2 --name nop \
--kernel nop.bin --debug-single-step
#grep on rip: will give
# no VM exit expected for RIP=0
# rip: 0000000000000003 rsp: 0000000000008000 flags: 0000000000000002
# rip: 0000000000000005 rsp: 0000000000008000 flags: 0000000000000002
# no VM exit for RIP=7! but ax is properly set at 2
# rip: 000000000000000a rsp: 0000000000008000 flags: 0000000000000002
# no VM exit for RIP=c! but ax is properly set at 3
# rip: 000000000000000f rsp: 0000000000008000 flags: 0000000000000002
