From mboxrd@z Thu Jan  1 00:00:00 1970
From: Sean Christopherson <sean.j.christopherson@intel.com>
Subject: Re: [PATCH] kvm: add proper frame pointer logic for vmx
Date: Tue, 15 Jan 2019 09:48:45 -0800
Message-ID: <20190115174845.GB21622@linux.intel.com>
References: <20190115064459.70513-1-cai@lca.pw>
 <a6477022-bbc0-fc00-2c8e-75007b468d3f@lca.pw>
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Cc: Paolo Bonzini <pbonzini@redhat.com>, rkrcmar@redhat.com,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org
To: Qian Cai <cai@lca.pw>
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On Tue, Jan 15, 2019 at 11:43:20AM -0500, Qian Cai wrote:
> 
> 
> 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.

What is ".part." and where does it come from?  Searching for information
is futile, the term is too generic.

> 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().