Re: [PATCH] KVM: arm64: Disable LTO in hyp

[Marc Zyngier]

On Fri, 05 Mar 2021 02:38:17 +0000,
Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
> 
> On Thu, Mar 4, 2021 at 2:34 PM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
> >
> > On Thu, Mar 4, 2021 at 2:17 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > >
> > > On Thu, 04 Mar 2021 21:25:41 +0000,
> > > Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:

[...]

> > > > I assume hyp_panic() ends up being placed too far from __guest_enter()
> > > > when the kernel is large enough. Possibly something to do with LLVM
> > > > always splitting functions into separate sections with LTO. I'm not
> > > > sure why the linker cannot shuffle things around to make everyone
> > > > happy in this case, but I confirmed that this patch also fixes the
> > > > build issue for me:
> > > >
> > > > diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
> > > > index af8e940d0f03..128197b7c794 100644
> > > > --- a/arch/arm64/kvm/hyp/vhe/switch.c
> > > > +++ b/arch/arm64/kvm/hyp/vhe/switch.c
> > > > @@ -214,7 +214,7 @@ static void __hyp_call_panic(u64 spsr, u64 elr, u64 par)
> > > >  }
> > > >  NOKPROBE_SYMBOL(__hyp_call_panic);
> > > >
> > > > -void __noreturn hyp_panic(void)
> > > > +void __noreturn hyp_panic(void) __section(".text")
> > > >  {
> > > >         u64 spsr = read_sysreg_el2(SYS_SPSR);
> > > >         u64 elr = read_sysreg_el2(SYS_ELR);
> > > >
> > >
> > > We're getting into black-magic territory here. Why wouldn't hyp_panic
> > > be in the .text section already?
> >
> > It's not quite black magic. LLVM essentially flips on
> > -ffunction-sections with LTO and therefore, hyp_panic() will be in
> > .text.hyp_panic in vmlinux.o, while __guest_enter() will be in .text.
> > Everything ends up in .text when we link vmlinux, of course.
> >
> > $ readelf --sections vmlinux.o | grep hyp_panic
> >   [3936] .text.hyp_panic   PROGBITS         0000000000000000  004b56e4
> 
> Note that disabling LTO here has essentially the same effect as using
> __section(".text"). It stops the compiler from splitting these
> functions into .text.* sections and makes it less likely that
> hyp_panic() ends up too far away from __guest_enter().
> 
> If neither of these workarounds sound appealing, I suppose we could
> alternatively change hyp/entry.S to use adr_l for hyp_panic. Thoughts?

That would be an actual fix instead of a workaround, as it would
remove existing assumptions about the relative locations of the two
objects. I guess you need to fix both instances with something such
as:

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index b0afad7a99c6..a43e1f7ee354 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -85,8 +85,10 @@ SYM_INNER_LABEL(__guest_exit_panic, SYM_L_GLOBAL)
 
 	// If the hyp context is loaded, go straight to hyp_panic
 	get_loaded_vcpu x0, x1
-	cbz	x0, hyp_panic
-
+	cbnz	x0, 1f
+	adr_l	x0, hyp_panic
+	br	x0
+1:
 	// The hyp context is saved so make sure it is restored to allow
 	// hyp_panic to run at hyp and, subsequently, panic to run in the host.
 	// This makes use of __guest_exit to avoid duplication but sets the
@@ -94,7 +96,7 @@ SYM_INNER_LABEL(__guest_exit_panic, SYM_L_GLOBAL)
 	// current state is saved to the guest context but it will only be
 	// accurate if the guest had been completely restored.
 	adr_this_cpu x0, kvm_hyp_ctxt, x1
-	adr	x1, hyp_panic
+	adr_l	x1, hyp_panic
 	str	x1, [x0, #CPU_XREG_OFFSET(30)]
 
 	get_vcpu_ptr	x1, x0

which is completely untested. I wouldn't be surprised if there were
more of these somewhere.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.