linux-kernel - Re: [PATCH] KVM: selftests: Don't skip L2's VMCALL in SMM test for SVM guest

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Date:   Wed, 26 Jan 2022 16:23:02 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH] KVM: selftests: Don't skip L2's VMCALL in SMM test for
 SVM guest

Sean Christopherson <seanjc@...gle.com> writes:

> Don't skip the vmcall() in l2_guest_code() prior to re-entering L2, doing
> so will result in L2 running to completion, popping '0' off the stack for
> RET, jumping to address '0', and ultimately dying with a triple fault
> shutdown.
>
> It's not at all obvious why the test re-enters L2 and re-executes VMCALL,
> but presumably it serves a purpose.  

I managed to forget everything but it seems my intentions were to test
two things:

- "Enter SMM during L2 execution and check that we correctly return from
  it."
- "Perform save/restore while the guest is in SMM triggered during L2
  execution" 

the later could've been complemented with "and try running L2 after".

> The VMX path doesn't skip vmcall(), and the test can't possibly have
> passed on SVM

Well, it kind of works for me (pre-patch) :-) I do see #DF in the trace
but not #TF.

>, so just do what VMX does.
>

Makes sense. I can't recall how "+= 3" appeared.

> Fixes: d951b2210c1a ("KVM: selftests: smm_test: Test SMM enter from L2")
> Cc: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  tools/testing/selftests/kvm/x86_64/smm_test.c | 1 -
>  1 file changed, 1 deletion(-)
>
> diff --git a/tools/testing/selftests/kvm/x86_64/smm_test.c b/tools/testing/selftests/kvm/x86_64/smm_test.c
> index 2da8eb8e2d96..a626d40fdb48 100644
> --- a/tools/testing/selftests/kvm/x86_64/smm_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/smm_test.c
> @@ -105,7 +105,6 @@ static void guest_code(void *arg)
>  
>  		if (cpu_has_svm()) {
>  			run_guest(svm->vmcb, svm->vmcb_gpa);
> -			svm->vmcb->save.rip += 3;
>  			run_guest(svm->vmcb, svm->vmcb_gpa);
>  		} else {
>  			vmlaunch();
>
> base-commit: e2e83a73d7ce66f62c7830a85619542ef59c90e4

Reviewed-and-tested-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly