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

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Date:   Thu, 21 Jan 2021 19:00:45 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     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, 2021-01-15 at 08:30 -0800, Sean Christopherson wrote:
> 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

I think that this is a good idea, but should be done in a separate patch.

> 
> 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.
> 

As I said the kvm tracing has a lot of things that can be imporoved, 
and as it is often the only way to figure out complex bugs as these I had to deal with recently,
I will do more improvements in this area as time permits.

Best regards,
	Maxim Levitsky