linux-kernel - Re: [PATCH 6/7] KVM: SVM: move MSR_IA32_SPEC

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Message-ID: <cd7fcdec-bf2c-ca27-4355-ce56ab9538d9@redhat.com>
Date:   Wed, 2 Nov 2022 17:02:36 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        jmattson@...gle.com, seanjc@...gle.com
Subject: Re: [PATCH 6/7] KVM: SVM: move MSR_IA32_SPEC_CTRL save/restore to
 assembly

On 11/2/22 16:28, Josh Poimboeuf wrote:
> On Fri, Oct 28, 2022 at 07:07:22PM -0400, Paolo Bonzini wrote:
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -3918,10 +3918,21 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu)
>>   	struct vcpu_svm *svm = to_svm(vcpu);
>>   	unsigned long vmcb_pa = svm->current_vmcb->pa;
>>   
>> +	/*
>> +	 * For non-nested case:
>> +	 * If the L01 MSR bitmap does not intercept the MSR, then we need to
>> +	 * save it.
>> +	 *
>> +	 * For nested case:
>> +	 * If the L02 MSR bitmap does not intercept the MSR, then we need to
>> +	 * save it.
>> +	 */
>> +	bool spec_ctrl_intercepted = msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL);
> 
> This triggers a warning:
> 
>    vmlinux.o: warning: objtool: svm_vcpu_enter_exit+0x3d: call to svm_msrpm_offset() leaves .noinstr.text section
> 
> svm_vcpu_enter_exit() is noinstr, but it's calling
> msr_write_intercepted() which is not.

I suspect I didn't see it because it's inlined here, but it has to be 
fixed indeed.

> That's why in the VMX code I did the call to msr_write_intercepted() (in
> __vmx_vcpu_run_flags) before calling vmx_vcpu_enter_exit().

Yes, it's easy to do the same and do it in svm_vcpu_run().

Paolo