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

[Lai Jiangshan <laijs@linux.alibaba.com>]

On 2021/7/9 17:49, Paolo Bonzini wrote:
> 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.
> 

Hello Paolo

I just remembered I once came across the patch, but I forgot it when
I wrote the mail.

It seems kvm_vcpu_check_breakpoint() handles only for code breakpoint
and doesn't handle for data breakpoints.

And no code handles DR7_GD bit when the emulation is not resulted from
vm-exit. (for example, the non-first instruction when kvm emulates
instructions back to back and the instruction accesses to DBn).

Thanks
Lai


> 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.