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

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Date:   Tue, 15 Jan 2019 13:29:22 -0500
From:   Qian Cai <cai@....pw>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
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 1/15/19 12:49 PM, Sean Christopherson wrote:
> 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.

For example, this works too,

diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile
index 69b3a7c30013..990dfc254e71 100644
--- a/arch/x86/kvm/Makefile
+++ b/arch/x86/kvm/Makefile
@@ -4,7 +4,7 @@ ccflags-y += -Iarch/x86/kvm

 CFLAGS_x86.o := -I.
 CFLAGS_svm.o := -I.
-CFLAGS_vmx.o := -I.
+CFLAGS_vmx.o := -I. -fdisable-tree-fnsplit

 KVM := ../../../virt/kvm