linux-kernel - Re: [PATCH] kvm: add proper frame pointer logic for vmx

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Date:   Tue, 15 Jan 2019 09:49:53 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Qian Cai <cai@....pw>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.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 Tue, Jan 15, 2019 at 09:48:45AM -0800, Sean Christopherson wrote:
> 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.

And never mind, my eyes glazed over -fdiable-ipa-fnsplit.

> > 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().