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Date:   Tue, 15 Jan 2019 11:43:20 -0500
From:   Qian Cai <cai@....pw>
To:     Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.com
Cc:     sean.j.christopherson@...el.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: add proper frame pointer logic for vmx



On 1/15/19 2:13 AM, Paolo Bonzini wrote:
> Hmm, maybe like this:
> 
> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index bcef2c7e9bc4..33122fa9d4bd 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S
> @@ -26,19 +26,17 @@ ENTRY(vmx_vmenter)
>  	ret
> 
>  2:	vmlaunch
> +3:
>  	ret
> 
> -3:	cmpb $0, kvm_rebooting
> -	jne 4f
> -	call kvm_spurious_fault
> -4:	ret
> -
>  	.pushsection .fixup, "ax"
> -5:	jmp 3b
> +4:	cmpb $0, kvm_rebooting
> +	jne 3b
> +	jmp kvm_spurious_fault
>  	.popsection
> 
> -	_ASM_EXTABLE(1b, 5b)
> -	_ASM_EXTABLE(2b, 5b)
> +	_ASM_EXTABLE(1b, 4b)
> +	_ASM_EXTABLE(2b, 4b)
> 
>  ENDPROC(vmx_vmenter)

No, that will not work. The problem is in vmx.o where I just sent another patch
for it.

I can see there are five options to solve it.

1) always inline vmx_vcpu_run()
2) always noinline vmx_vcpu_run()
3) add -fdiable-ipa-fnsplit option to Makefile for vmx.o
4) let STACK_FRAME_NON_STANDARD support part.* syntax.
5) trim-down vmx_vcpu_run() even more to not causing splitting by GCC.

Option 1) and 2) seems give away the decision for user with
CONFIG_CC_OPTIMIZE_FOR_(PERFORMANCE/SIZE).

Option 3) prevents other functions there for splitting for optimization.

Option 4) and 5) seems tricky to implement.

I am not more leaning to 3) as only other fuction will miss splitting is
vmx_segment_access_rights().

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