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

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Message-ID: <17c961e9-bc76-9fe0-b759-3b427f422bbc@lca.pw>
Date:   Tue, 15 Jan 2019 12:31:34 -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 11:43 AM, 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.
> 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().
> 

Option 3) seems a bit tricky to implement too, as it needs to check for the
compiler version before to see if the option is available before put it to the
CFLAGS.