Re: [PATCH] KVM: VMX: Update instruction length on intercepted BP

[Jan Kiszka <jan.kiszka@siemens.com>]

Jan Kiszka wrote:
> Gleb Natapov wrote:
>> Lets check if SVM works. I can do that if you tell me how.
> 
> - Fire up some Linux guest with gdb installed
> - Attach gdb to gdbstub of the VM
> - Set a soft breakpoint in guest kernel, ideally where it does not
>   immediately trigger, e.g. on sys_reboot (use grep sys_reboot
>   /proc/kallsyms if you don't have symbols for the guest kernel)
> - Start gdb /bin/true in the guest
> - run
> 
> As gdb sets some automatic breakpoints, this already exercises the
> reinjection of #BP.

I just did this on our primary AMD platform (Embedded Opteron, 13KS EE),
and it just worked.

But this is a fairly new processor. Consequently, it reports NextRIP
support via cpuid function 0x8000000A. Looking for an older one too.

In the meantime I also browsed a bit more in the manuals, and I don't
think stepping over or (what is actually required) into an INT3 will
work. We can't step into as the processor clears TF on any event handler
entry. And stepping over would cause troubles

a) as an unknown amount of code may run without #DB interception
b) we would fiddle with TF in code that is already under debugger
   control, thus we would very likely run into conflicts.

Leaves us with tricky INT3 emulation. Sigh.

Jan

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