linux-kernel - Re: [PATCH v2 2/3] KVM: nVMX: add kvm_nested_vmlaunch

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Date:   Fri, 15 Jan 2021 08:30:24 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 2/3] KVM: nVMX: add kvm_nested_vmlaunch_resume
 tracepoint

On Fri, Jan 15, 2021, Paolo Bonzini wrote:
> On 15/01/21 01:14, Sean Christopherson wrote:
> > > +	trace_kvm_nested_vmlaunch_resume(kvm_rip_read(vcpu),
> > Hmm, won't this RIP be wrong for the migration case?  I.e. it'll be L2, not L1
> > as is the case for the "true" nested VM-Enter path.
> 
> It will be the previous RIP---might as well be 0xfffffff0 depending on what
> userspace does.  I don't think you can do much better than that, using
> vmcs12->host_rip would be confusing in the SMM case.
> 
> > > +					 vmx->nested.current_vmptr,
> > > +					 vmcs12->guest_rip,
> > > +					 vmcs12->vm_entry_intr_info_field);
> > The placement is a bit funky.  I assume you put it here so that calls from
> > vmx_set_nested_state() also get traced.  But, that also means
> > vmx_pre_leave_smm() will get traced, and it also creates some weirdness where
> > some nested VM-Enters that VM-Fail will get traced, but others will not.
> > 
> > Tracing vmx_pre_leave_smm() isn't necessarily bad, but it could be confusing,
> > especially if the debugger looks up the RIP and sees RSM.  Ditto for the
> > migration case.
> 
> Actually tracing vmx_pre_leave_smm() is good, and pointing to RSM makes
> sense so I'm not worried about that.

Ideally there would something in the tracepoint to differentiate the various
cases.  Not that the RSM/migration cases will pop up often, but I think it's an
easily solved problem that could avoid confusion.

What if we captured vmx->nested.smm.guest_mode and from_vmentry, and explicitly
record what triggered the entry?

	TP_printk("from: %s rip: 0x%016llx vmcs: 0x%016llx nrip: 0x%016llx intr_info: 0x%08x",
		  __entry->vmenter ? "VM-Enter" : __entry->smm ? "RSM" : "SET_STATE",
		  __entry->rip, __entry->vmcs, __entry->nested_rip,
		  __entry->entry_intr_info

Side topic, can we have an "official" ruling on whether KVM tracepoints should
use colons and/or commas? And probably same question for whether or not to
prepend zeros.  E.g. kvm_entry has "vcpu %u, rip 0x%lx" versus "rip: 0x%016llx
vmcs: 0x%016llx".  It bugs me that we're so inconsistent.