linux-kernel - Re: [PATCH v8 4/4] kvm: vmx: virtualize split lock detection

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Message-ID: <871rooodad.fsf@nanos.tec.linutronix.de>
Date:   Wed, 15 Apr 2020 19:43:22 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH v8 4/4] kvm: vmx: virtualize split lock detection

Xiaoyao Li <xiaoyao.li@...el.com> writes:
> +/*
> + * Note: for guest, feature split lock detection can only be enumerated through
> + * MSR_IA32_CORE_CAPABILITIES bit. The FMS enumeration is unsupported.

That comment is confusing at best.

> + */
> +static inline bool guest_cpu_has_feature_sld(struct kvm_vcpu *vcpu)
> +{
> +	return vcpu->arch.core_capabilities &
> +	       MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT;
> +}
> +
> +static inline bool guest_cpu_sld_on(struct vcpu_vmx *vmx)
> +{
> +	return vmx->msr_test_ctrl & MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +}
> +
> +static inline void vmx_update_sld(struct kvm_vcpu *vcpu, bool on)
> +{
> +	/*
> +	 * Toggle SLD if the guest wants it enabled but its been disabled for
> +	 * the userspace VMM, and vice versa.  Note, TIF_SLD is true if SLD has
> +	 * been turned off.  Yes, it's a terrible name.

Instead of writing that useless blurb you could have written a patch
which changes TIF_SLD to TIF_SLD_OFF to make it clear.

> +	 */
> +	if (sld_state == sld_warn && guest_cpu_has_feature_sld(vcpu) &&
> +	    on == test_thread_flag(TIF_SLD)) {
> +		    sld_update_msr(on);
> +		    update_thread_flag(TIF_SLD, !on);

Of course you completely fail to explain why TIF_SLD needs to be fiddled
with.

> @@ -1188,6 +1217,10 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
>  #endif
>
> 	vmx_set_host_fs_gs(host_state, fs_sel, gs_sel, fs_base, gs_base);
> +
> +	vmx->host_sld_on = !test_thread_flag(TIF_SLD);

This inverted storage is non-intuitive. What's wrong with simply
reflecting the TIF_SLD state?

> +	vmx_update_sld(vcpu, guest_cpu_sld_on(vmx));
> +
>	vmx->guest_state_loaded = true;
> }
>
> @@ -1226,6 +1259,9 @@ static void vmx_prepare_switch_to_host(struct vcpu_vmx *vmx)
> 	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
>  #endif
> 	load_fixmap_gdt(raw_smp_processor_id());
> +
> +	vmx_update_sld(&vmx->vcpu, vmx->host_sld_on);
> +

vmx_prepare_switch_to_guest() is called via:

kvm_arch_vcpu_ioctl_run()
  vcpu_run()
    vcpu_enter_guest()
      preempt_disable();
      kvm_x86_ops.prepare_guest_switch(vcpu);

but vmx_prepare_switch_to_host() is invoked at the very end of:

kvm_arch_vcpu_ioctl_run()
  .....
  vcpu_run()
  .....
  vcpu_put()
    vmx_vcpu_put()
      vmx_prepare_switch_to_host();

That asymmetry does not make any sense without an explanation.

What's even worse is that vmx_prepare_switch_to_host() is invoked with
preemption enabled, so MSR state and TIF_SLD state can get out of sync
on preemption/migration.

> @@ -1946,9 +1992,15 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> 
> 	switch (msr_index) {
> 	case MSR_TEST_CTRL:
> -		if (data)
> +		if (data & ~vmx_msr_test_ctrl_valid_bits(vcpu))
> 			return 1;
> 
> +		vmx->msr_test_ctrl = data;
> +
> +		preempt_disable();

This preempt_disable/enable() lacks explanation as well.

> +		if (vmx->guest_state_loaded)
> +			vmx_update_sld(vcpu, guest_cpu_sld_on(vmx));
> +		preempt_enable();

How is updating msr_test_ctrl valid if this is invoked from the IOCTL,
i.e. host_initiated == true?

That said, I also hate the fact that you export both the low level MSR
function _and_ the state variable. Having all these details including the
TIF mangling in the VMX code is just wrong.

Thanks,

        tglx