Re: [PATCH] KVM: X86: Also reload the debug registers before kvm_x86->run() when the host is using them

[Paolo Bonzini]

On 09/07/21 05:09, Lai Jiangshan wrote:
> I just noticed that emulation.c fails to emulate with DBn.
> Is there any problem around it?

Just what you said, it's not easy and the needs are limited.  I 
implemented kvm_vcpu_check_breakpoint because I was interested in using 
hardware breakpoints from gdb, even with unrestricted_guest=0 and 
invalid guest state, but that's it.

Paolo

> For code breakpoint, if the instruction didn't cause vm-exit,
> (for example, the 2nd instruction when kvm emulates instructions
> back to back) emulation.c fails to emulate with DBn.
>
> For code breakpoint, if the instruction just caused vm-exit.
> It is difficult to analyze this case due to the complex priorities
> between vectored events and fault-like vm-exit.
> Anyway, if it is an instruction that vm-exit has priority over #DB,
> emulation.c fails to emulate with DBn.
>
> For data breakpoint, a #DB must be delivered to guest or to VMM (when
> guest-debug) after the instruction. But emulation.c doesn't do so.
>
> And the existence of both of effective DBn (guest debug) and guest DBn
> complicates the problem when we try to emulate them.