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SubjectRe: [PATCH] kvm: add proper frame pointer logic for vmx
On Tue, Jan 15, 2019 at 11:06:17AM -0800, Sean Christopherson wrote:
> > 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
> >
> > 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.

Do you mean moving the asm blob to a .S file instead of inline asm? If
so, I think that's definitely a good idea. It would be a nice cleanup,
regardless of the objtool false positive.

That would allow vmx_vcpu_run() to be a "normal" C function which
objtool can validate (and also create ORC data for). It would also
prevent future nasty GCC optimizations (which was why the __noclone was
needed in the first place).

And also, I *think* objtool would no longer warn in that case, because
there would no longer be any calls in the function after popping %rbp.
Though if I'm wrong about that, I'd be glad to help fix the warning one
way or another.


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