linux-kernel - Re: [PATCH v7 8/8] KVM: VMX: enable IPI virtualization

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Message-ID: <YlmCxOcWvbXzaYpw@google.com>
Date:   Fri, 15 Apr 2022 14:35:48 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Zeng Guang <guang.zeng@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kim Phillips <kim.phillips@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jethro Beekman <jethro@...tanix.com>,
        "Huang, Kai" <kai.huang@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Hu, Robert" <robert.hu@...el.com>,
        "Gao, Chao" <chao.gao@...el.com>
Subject: Re: [PATCH v7 8/8] KVM: VMX: enable IPI virtualization

On Sat, Apr 09, 2022, Zeng Guang wrote:
> 
> On 4/5/2022 1:57 AM, Sean Christopherson wrote:
> > On Sun, Apr 03, 2022, Zeng Guang wrote:
> > > On 4/1/2022 10:37 AM, Sean Christopherson wrote:
> > > > > @@ -4219,14 +4226,21 @@ static void vmx_refresh_apicv_exec_ctrl(struct kvm_vcpu *vcpu)
> > > > >    	pin_controls_set(vmx, vmx_pin_based_exec_ctrl(vmx));
> > > > >    	if (cpu_has_secondary_exec_ctrls()) {
> > > > > -		if (kvm_vcpu_apicv_active(vcpu))
> > > > > +		if (kvm_vcpu_apicv_active(vcpu)) {
> > > > >    			secondary_exec_controls_setbit(vmx,
> > > > >    				      SECONDARY_EXEC_APIC_REGISTER_VIRT |
> > > > >    				      SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY);
> > > > > -		else
> > > > > +			if (enable_ipiv)
> > > > > +				tertiary_exec_controls_setbit(vmx,
> > > > > +						TERTIARY_EXEC_IPI_VIRT);
> > > > > +		} else {
> > > > >    			secondary_exec_controls_clearbit(vmx,
> > > > >    					SECONDARY_EXEC_APIC_REGISTER_VIRT |
> > > > >    					SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY);
> > > > > +			if (enable_ipiv)
> > > > > +				tertiary_exec_controls_clearbit(vmx,
> > > > > +						TERTIARY_EXEC_IPI_VIRT);
> > > > Oof.  The existing code is kludgy.  We should never reach this point without
> > > > enable_apicv=true, and enable_apicv should be forced off if APICv isn't supported,
> > > > let alone seconary exec being support.
> > > > 
> > > > Unless I'm missing something, throw a prep patch earlier in the series to drop
> > > > the cpu_has_secondary_exec_ctrls() check, that will clean this code up a smidge.
> > > cpu_has_secondary_exec_ctrls() check can avoid wrong vmcs write in case mistaken
> > > invocation.
> > KVM has far bigger problems on buggy invocation, and in that case the resulting
> > printk + WARN from the failed VMWRITE is a good thing.
> 
> SDM doesn't define VMWRITE failure for such case.

Yes it absolutely does.  cpu_has_secondary_exec_ctrls() checks if the VMCS field
_exists_, not if it's being used by KVM (though that's a moot point since KVM
always enables secondary controls when it's supported).  VMWRITE to non-existent
fields cause VM-Fail.

  ELSIF secondary source operand does not correspond to any VMCS field
  THEN VMfailValid(VMREAD/VMWRITE from/to unsupported VMCS component);