On 16/02/2017 15:52, Francois Ozog wrote: > 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. This should be fixed in kernel 4.10. Thanks, Paolo > 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 >
