linux-kernel - Re: [RFC PATCH 06/18] KVM: VMX: Wire up Intel MBEC enable/disable logic

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Message-ID: <aCNI72KuMLfWb9F2@google.com>
Date: Tue, 13 May 2025 06:28:15 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jon Kohler <jon@...anix.com>
Cc: "pbonzini@...hat.com" <pbonzini@...hat.com>, "tglx@...utronix.de" <tglx@...utronix.de>, 
	"mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "x86@...nel.org" <x86@...nel.org>, 
	"hpa@...or.com" <hpa@...or.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 06/18] KVM: VMX: Wire up Intel MBEC enable/disable logic

On Tue, May 13, 2025, Jon Kohler wrote:
> > On May 12, 2025, at 2:23 PM, Sean Christopherson <seanjc@...gle.com> wrote:
> > > On Thu, Mar 13, 2025, Jon Kohler wrote:
> >> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> >> index 7a98f03ef146..116910159a3f 100644
> >> --- a/arch/x86/kvm/vmx/vmx.c
> >> +++ b/arch/x86/kvm/vmx/vmx.c
> >> @@ -2694,6 +2694,7 @@ static int setup_vmcs_config(struct vmcs_config *vmcs_conf,
> >> return -EIO;
> >> 
> >> vmx_cap->ept = 0;
> >> + _cpu_based_2nd_exec_control &= ~SECONDARY_EXEC_MODE_BASED_EPT_EXEC;
> >> _cpu_based_2nd_exec_control &= ~SECONDARY_EXEC_EPT_VIOLATION_VE;
> >> }
> >> if (!(_cpu_based_2nd_exec_control & SECONDARY_EXEC_ENABLE_VPID) &&
> >> @@ -4641,11 +4642,15 @@ static u32 vmx_secondary_exec_control(struct vcpu_vmx *vmx)
> >> exec_control &= ~SECONDARY_EXEC_ENABLE_VPID;
> >> if (!enable_ept) {
> >> exec_control &= ~SECONDARY_EXEC_ENABLE_EPT;
> >> + exec_control &= ~SECONDARY_EXEC_MODE_BASED_EPT_EXEC;
> >> exec_control &= ~SECONDARY_EXEC_EPT_VIOLATION_VE;
> >> enable_unrestricted_guest = 0;
> >> }
> >> if (!enable_unrestricted_guest)
> >> exec_control &= ~SECONDARY_EXEC_UNRESTRICTED_GUEST;
> >> + if (!enable_pt_guest_exec_control)
> >> + exec_control &= ~SECONDARY_EXEC_MODE_BASED_EPT_EXEC;
> > 
> > This is wrong and unnecessary.  As mentioned early, the input that matters is
> > vmcs12.  This flag should *never* be set for vmcs01.
> 
> I’ll page this back in, but I’m like 75% sure it didn’t work when I did it that way.

Then you had other bugs.  The control is per-VMCS and thus needs to be emulated
as such.  Definitely holler if you get stuck, there's no need to develop this in
complete isolation.