linux-kernel - Re: [PATCH 19/61] KVM: VMX: Add helpers to query Intel PT mode

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Date:   Mon, 24 Feb 2020 14:18:07 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 19/61] KVM: VMX: Add helpers to query Intel PT mode

On Fri, Feb 21, 2020 at 04:16:31PM +0100, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@...el.com> writes:
> 
> > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> > index a4f7f737c5d4..70eafa88876a 100644
> > --- a/arch/x86/kvm/vmx/vmx.h
> > +++ b/arch/x86/kvm/vmx/vmx.h
> > @@ -449,7 +449,7 @@ static inline void vmx_segment_cache_clear(struct vcpu_vmx *vmx)
> >  static inline u32 vmx_vmentry_ctrl(void)
> >  {
> >  	u32 vmentry_ctrl = vmcs_config.vmentry_ctrl;
> > -	if (pt_mode == PT_MODE_SYSTEM)
> > +	if (vmx_pt_mode_is_system())
> 
> Just wondering, would it rather be better to say
>         if (!vmx_pt_supported())
> here?
> 
> >  		vmentry_ctrl &= ~(VM_ENTRY_PT_CONCEAL_PIP |
> >  				  VM_ENTRY_LOAD_IA32_RTIT_CTL);
> >  	/* Loading of EFER and PERF_GLOBAL_CTRL are toggled dynamically */
> > @@ -460,7 +460,7 @@ static inline u32 vmx_vmentry_ctrl(void)
> >  static inline u32 vmx_vmexit_ctrl(void)
> >  {
> >  	u32 vmexit_ctrl = vmcs_config.vmexit_ctrl;
> > -	if (pt_mode == PT_MODE_SYSTEM)
> > +	if (vmx_pt_mode_is_system())
> 
> ... and here? I.e. to cover the currently unsupported 'host-only' mode.

Hmm, good question.  I don't think so?  On VM-Enter, RTIT_CTL would need to
be loaded to disable PT.  Clearing RTIT_CTL on VM-Exit would be redundant
at that point[1].  And AIUI, the PIP for VM-Enter/VM-Exit isn't needed
because there is no context switch from the decoder's perspective.

Note, the original upstreaming series also used "pt_mode == PT_MODE_SYSTEM"
logic for this check when "host-only mode" was supported[2].

[1] Arguably, KVM should use the VM-Exit MSR load list to atomically
    reenable tracing, but that's feedback for a non-existence patch :-).
[2] https://patchwork.kernel.org/patch/10104533/

> 
> >  		vmexit_ctrl &= ~(VM_EXIT_PT_CONCEAL_PIP |
> >  				 VM_EXIT_CLEAR_IA32_RTIT_CTL);
> >  	/* Loading of EFER and PERF_GLOBAL_CTRL are toggled dynamically */