Re: Multiplexing RFLAGS.TF

[Jan Kiszka <jan.kiszka@web.de>]

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Am 29.07.2010 10:37, Avi Kivity wrote:
>  static int db_interception(struct vcpu_svm *svm)
> {
>     struct kvm_run *kvm_run = svm->vcpu.run;
> 
>     if (!(svm->vcpu.guest_debug &
>           (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) &&
>         !svm->nmi_singlestep) {
>         kvm_queue_exception(&svm->vcpu, DB_VECTOR);
>         return 1;
>     }
> 
>     if (svm->nmi_singlestep) {
>         svm->nmi_singlestep = false;
>         if (!(svm->vcpu.guest_debug & KVM_GUESTDBG_SINGLESTEP))
>             svm->vmcb->save.rflags &=
>                 ~(X86_EFLAGS_TF | X86_EFLAGS_RF);
>         update_db_intercept(&svm->vcpu);
>     }
> 
> This code assumes that either the guest is debugging itself, or
> (nmi_singlestep | guest debugging).  However if the guest is debugging
> itself and takes an NMI, or if both host and guest are debugging the
> guest, things will go wrong.

I know.

> 
> So we need an rflags_guest_owned_bits, usually set to -1ULL, but
> sometimes (NMI, host debugging) clearing EFLAGS_TF.  When we do that, we
> need to intercept instructions that influence RFLAGS.TF (POPF, IRET,
> INTn) and emulate them.  Otherwise, the guest can disable tracing which
> was enabled on behalf of the host.

I was still waiting on some smart idea from AMD how to properly
implement NMIs without having to fully emulate IRET. Probably there is
no alternative...

> 
> We also need to drop the 'return 1' on the top of the function to allow
> both guest and host tracing.

Support for host and guest-initiated tracing at the same time would be
nice, but I would not spend to much effort on this corner case of the
corner cases. If it happens to fall off from the NMI fix, OK. But
otherwise let the host rule TF if it wants to.

> 
> On Intel, the situation is harder.  We can't trap POPF or IRET.  What we
> can do, is use the Monitor Trap Flag on hosts that have it.

Setting TF before POPF and IRET should give us at least the chance to
provide host-overrules-guest tracing support. Adding monitor trap
support would be nice. It would allow more things actually, but it may
then require some additional knob in the user/kernel interface to
control the mode (MTF steps into exceptions/interrupts, TF not).

> 
> Comments?  Perhaps I missed something.  Maybe I'll try writing a test
> case to prove the brokenness, it's fashionable these days.
> 
> Jan, as this is your code, are you interested in doing this?

I'm not very keen on writing complex and error-prone opcode emulations,
but in principle resolving the AMD issue is on my long to-do list - with
moderate prio though.

Cheers from Lancaster County PA,
Jan


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