On Fri, Mar 5, 2021 at 6:22 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
On Fri, 5 Mar 2021 at 12:38, Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> 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:
Agree with replacing the conditional branches that refer to external
symbols: the compiler never emits those, for the reason we are seeing
here, i.e., the range is simply insufficient.
But let's just use 'b hyp_panic' instead, no?
Alright, this seems to work for me:
diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index b0afad7a99c6..c62265951467 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
+ b hyp_panic
+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
But when I say work, I mean this fixes the allmodconfig build with
LTO, and my kernel boots at EL2. I don't actually have a way to
properly test KVM on arm64. If nobody sees obvious issues here, I can
send a proper patch a bit later.
