Re: [PATCH v2] KVM: arm64: Initialize VCPU mdcr_el2 before loading it

[Marc Zyngier <maz@kernel.org>]

On Tue, 30 Mar 2021 18:49:54 +0100,
Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> Hi Marc,
> 
> On 3/30/21 6:13 PM, Alexandru Elisei wrote:
> > [..]
> >>> +}
> >>> +
> >>>  /**
> >>>   * kvm_arm_reset_debug_ptr - reset the debug ptr to point to the vcpu state
> >>>   */
> >>> @@ -83,12 +137,7 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
> >>>   * @vcpu:	the vcpu pointer
> >>>   *
> >>>   * This is called before each entry into the hypervisor to setup any
> >>> - * debug related registers. Currently this just ensures we will trap
> >>> - * access to:
> >>> - *  - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
> >>> - *  - Debug ROM Address (MDCR_EL2_TDRA)
> >>> - *  - OS related registers (MDCR_EL2_TDOSA)
> >>> - *  - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
> >>> + * debug related registers.
> >>>   *
> >>>   * Additionally, KVM only traps guest accesses to the debug registers if
> >>>   * the guest is not actively using them (see the KVM_ARM64_DEBUG_DIRTY
> >>> @@ -100,27 +149,14 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
> >>>  
> >>>  void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
> >>>  {
> >>> -	bool trap_debug = !(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY);
> >>>  	unsigned long mdscr, orig_mdcr_el2 = vcpu->arch.mdcr_el2;
> >>>  
> >>>  	trace_kvm_arm_setup_debug(vcpu, vcpu->guest_debug);
> >>>  
> >>> -	/*
> >>> -	 * This also clears MDCR_EL2_E2PB_MASK to disable guest access
> >>> -	 * to the profiling buffer.
> >>> -	 */
> >>> -	vcpu->arch.mdcr_el2 = __this_cpu_read(mdcr_el2) & MDCR_EL2_HPMN_MASK;
> >>> -	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
> >>> -				MDCR_EL2_TPMS |
> >>> -				MDCR_EL2_TPMCR |
> >>> -				MDCR_EL2_TDRA |
> >>> -				MDCR_EL2_TDOSA);
> >>> +	kvm_arm_setup_mdcr_el2(vcpu, __this_cpu_read(mdcr_el2));
> >>>  
> >>>  	/* Is Guest debugging in effect? */
> >>>  	if (vcpu->guest_debug) {
> >>> -		/* Route all software debug exceptions to EL2 */
> >>> -		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
> >>> -
> >>>  		/* Save guest debug state */
> >>>  		save_guest_debug_regs(vcpu);
> >>>  
> >>> @@ -174,7 +210,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
> >>>  
> >>>  			vcpu->arch.debug_ptr = &vcpu->arch.external_debug_state;
> >>>  			vcpu->arch.flags |= KVM_ARM64_DEBUG_DIRTY;
> >>> -			trap_debug = true;
> >> There is something that slightly worries me here: there is now a
> >> disconnect between flagging debug as dirty and setting the
> >> trapping. And actually, you now check for KVM_ARM64_DEBUG_DIRTY and
> >> set the trap bits *before* setting the dirty bit itself.
> >>
> >> Here, I believe you end up with guest/host confusion of breakpoints,
> >> which isn't great. Or did I miss something?
> > I'm sorry, but I don't understand what you mean. This is my understanding of what
> > is happening.
> >
> > Without this patch, trap_debug is set to true and the KVM_ARM64_DEBUG_DIRTY flag
> > is set if vcpu->guest_debug & KVM_GUESTDBG_USE_HW. Further down, trap debug is
> > only used when computing mdcr_el2.
> >
> > With this patch, trap_debug is set to true if vcpu->guest_debug &
> > KVM_GUESTDBG_USE_HW and it's also used for computing mdcr_el2, but this happens in
> > kvm_arm_setup_mdcr_el2(), which is called at the start of kvm_arm_setup_debug().
> > The KVM_ARM_DEBUG_DIRTY flags is still set in kvm_arm_setup_debug() if
> > vcpu->guest_debug & KVM_GUESTDBG_USE_HW, like before.
> >
> > The guest never runs with the value computed in kvm_vcpu_first_run_init() unless
> > it's identical with the value recomputed in kvm_arm_setup_debug().
> >
> > The only difference I see is that mdcr_el2 is computed at the start of
> > kvm_arm_setup_debug(). I get the feeling I'm also missing something.
> 
> I think I understand what you mean, you are worried that we won't
> set the bit in mdcr_el2 to trap debug in the same place where we set
> the debug dirty flag.

Yes, that's what I mean. The code is conceptually as such ATM:

	debug_trap = (something based on vcpu->flags);
	if (something else) {
		check stuff;
		vcpu->flags |= stuff;
		debug_trap = true;
	}

	if (debug_trap)
		set trap conditions;

You are turning this into:

	debug_trap = (something based on vcpu->flags);
	if (debug_trap) {
		set trap conditions;
	}
	if (something else) {
		check stuff;
		vcpu->flags |= stuff;
	}

which isn't the same thing. In your case, it probably works because of
KVM_GUESTDBG_USE_HW, but that's really hard to follow, and we have had
so many bugs in the debug code that it really needs to be kept as
stupid as possible.

> If that's the case, then I can move kvm_arm_setup_mdcr_el2 right
> after the BUG_ON() and remove the KVM_GUESTDBG_USE_HW check because
> the KVM_ARM_DEBUG_DIRTY would be already set.

Yes, I think that'd be better.

> Question though, if mdcr_el2 is tied to the debug dirty flag, we
> ignore the flag here (code without this patch):
> 
>     BUG_ON(!vcpu->guest_debug &&
>         vcpu->arch.debug_ptr != &vcpu->arch.vcpu_debug_state);
> 
>     /* Trap debug register access */
>     if (trap_debug)
>         vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
> 
>     /* If KDE or MDE are set, perform a full save/restore cycle. */
>     if (vcpu_read_sys_reg(vcpu, MDSCR_EL1) & (DBG_MDSCR_KDE | DBG_MDSCR_MDE))
>         vcpu->arch.flags |= KVM_ARM64_DEBUG_DIRTY;
> 
> I suppose there's something I don't understand yet about how this is
> supposed to work.

The idea (IIRC) is that if MDSCR_EL1.KDE or MDSCR_EL1.MDE are set,
that's because the guest is currently debugging, and that we are
better off saying that the debug state is dirty, forcing a
save/restore cycle on entry.

You may want to dig into the git history for more accurate
information...

Hope this helps,

	M.

-- 
Without deviation from the norm, progress is not possible.

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