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

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Message-ID: <18c386f2-a588-6324-fcde-d13b66f66d4f@redhat.com>
Date:   Fri, 15 Jan 2021 14:48:38 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...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 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.

Paolo