Re: [PATCH] KVM: x86: fix singlestepping over syscall

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2017-06-27 16:20 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
>
>
> On 27/06/2017 05:41, Wanpeng Li wrote:
>>> KVM emulates syscall so that it can trap 32-bit syscall on Intel processors.
>>
>> We have a discussion to not expose syscall/sysret to Intel 32-bit
>> guest two years ago. https://lkml.org/lkml/2015/11/19/225 The
>> syscall/sysret just makes sense against long mode instead of

s/long mode/64-bit mode

>> compatibility/legacy mode of Intel CPU. We will get a #UD in 32-bit
>> guest, and syscall emulation is introduced by commit 66bb2ccd (KVM:
>> x86 emulator: add syscall emulation) to handle it. So why we still
>> expose syscall/sysret to Intel 32-bit guest?
>
> Because you didn't post v2 of that patch, I guess. :)
>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 87d3cb901935..0e846f0cb83b 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -5313,6 +5313,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
>>>         kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
>>>
>>>         ctxt->eflags = kvm_get_rflags(vcpu);
>>> +       ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
>>> +
>>
>> I guess this is used for "the sysret is executed the #DB is taken "as
>> if" the syscall insn just completed", however, there is no sysret
>> emulation, so how the #DB is taken after the sysret?
>
> No, it's used for instructions other than syscall and sysret:
>
>> +               if (r == EMULATE_DONE &&
>> +                   (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
>> +                       kvm_vcpu_do_singlestep(vcpu, &r);
>
> syscall (and sysret if it were emulated) overwrite ctxt->tf with the
> value of TF at the end of the instruction.  Other instructions don't, so
> that singlestep depends on EFLAGS.TF before the instruction is executed.

Why sysret is not emulated since SDM said that it can incur a #UD if
not in 64-bit mode?

Regards,
Wanpeng Li



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