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SubjectRe: [PATCH] kvm: add proper frame pointer logic for vmx
+cc Josh, for real this time.

On Tue, Jan 15, 2019 at 11:06:17AM -0800, Sean Christopherson wrote:
> +cc Josh
>
> Josh,
>
> The issue we're encountering is that vmx_vcpu_run() is marked with
> STACK_FRAME_NON_STANDARD because %rbp is saved/modified/restored when
> transitioning to/from the guest. But vmx_vcpu_run() also happens to
> be a giant blob of code that gcc will sometime split into separate
> .part.* functions, which means that objtool's symbol lookup to whitelist
> the function doesn't work.
>
> 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.
> > 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 4) is the most correct, but "tricky" is an understatement. Unless
> Josh is willing to pick up the task it'll likely have to wait.
>
> There's actually a few more options:
>
> 6) Replace "pop %rbp" in the vmx_vmenter() asm blob with an open-coded
> equivalent, e.g. "mov [%rsp], %rbp; add $8, %rsp". This runs an end-
> around on objtool since objtool explicitly keys off "pop %rbp" and NOT
> "mov ..., %rbp" (which is probably an objtool checking flaw?").
>
> 7) Move the vmx_vmenter() asm blob and a few other lines of code into a
> separate helper, e.g. __vmx_vcpu_run(), and mark that as having a
> non-standard stack frame.
>
> Option 6) is an ugly (but amusing) hack.
>
> Arguably we should do option 7) regardless of whether or not objtool gets
> updated to support fnsplit behavior, as it it allows objtool to do proper
> stack checking on the bulk of vmx_vcpu_run(). And it's a pretty trivial
> change.
>
> I'll send a patch for option 7), which in theory should fix the warning
> for all .configs.

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