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

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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
> 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.

Paolo



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