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

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Date:   Thu, 21 Jan 2021 18:58:03 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.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 14:48 +0100, 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.

Actually in this case, the initial RIP of 0x000000000000fff0 will be printed
which isn't that bad.

A tracepoint in nested state load function would be very nice to add
to mark this explicitly. I'll do this later.

> 
> 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.
> 
> Paolo
> 

I agree with that and indeed this was my intention.

In fact I will change the svm's tracepoint to behave the same way
in the next patch series (I'll move it to enter_svm_guest_mode).

(When I wrote this patch I somehow thought that this is what SVM already does).

Best regards,
	Maxim Levitsky