Re: [PATCH V2 5/9] arm64: exception: handle instruction abort at current EL

[Marc Zyngier <marc.zyngier@arm.com>]

On Wed, 6 Apr 2016 15:36:00 -0600
"Baicar, Tyler" <tbaicar@codeaurora.org> wrote:

Hi Tyler,

> Hello Marc,
> 
> On 4/6/2016 9:36 AM, Marc Zyngier wrote:
> > On 06/04/16 16:12, Tyler Baicar wrote:
> >> Add a handler for instruction aborts at the current EL
> >> (ESR_ELx_EC_IABT_CUR) so they are no longer handled in el1_inv.
> >> This allows firmware first handling for possible SEA
> >> (Synchronous External Abort) caused instruction abort at
> >> current EL.
> >>
> >> Signed-off-by: Tyler Baicar <tbaicar@codeaurora.org>
> >> Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>
> >> ---
> >>   arch/arm64/kernel/entry.S | 19 +++++++++++++++++++
> >>   1 file changed, 19 insertions(+)
> >>
> >> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> >> index 12e8d2b..f257856 100644
> >> --- a/arch/arm64/kernel/entry.S
> >> +++ b/arch/arm64/kernel/entry.S
> >> @@ -336,6 +336,8 @@ el1_sync:
> >>   	lsr	x24, x1, #ESR_ELx_EC_SHIFT	// exception class
> >>   	cmp	x24, #ESR_ELx_EC_DABT_CUR	// data abort in EL1
> >>   	b.eq	el1_da
> >> +	cmp	x24, #ESR_ELx_EC_IABT_CUR	// instruction abort in EL1
> >> +	b.eq	el1_ia
> >>   	cmp	x24, #ESR_ELx_EC_SYS64		// configurable trap
> >>   	b.eq	el1_undef
> >>   	cmp	x24, #ESR_ELx_EC_SP_ALIGN	// stack alignment exception
> >> @@ -363,6 +365,23 @@ el1_da:
> >>   	// disable interrupts before pulling preserved data off the stack
> >>   	disable_irq
> >>   	kernel_exit 1
> >> +el1_ia:
> >> +	/*
> >> +	 * Instruction abort handling
> >> +	 */
> >> +	mrs	x0, far_el1
> >> +	enable_dbg
> >> +	// re-enable interrupts if they were enabled in the aborted context
> >> +	tbnz	x23, #7, 1f			// PSR_I_BIT
> >> +	enable_irq
> >> +1:
> >> +	orr	x1, x1, #1 << 24		// use reserved ISS bit for instruction aborts
> >> +	mov	x2, sp				// struct pt_regs
> >> +	bl	do_mem_abort
> >> +
> >> +	// disable interrupts before pulling preserved data off the stack
> >> +	disable_irq
> >> +	kernel_exit 1
> >>   el1_sp_pc:
> >>   	/*
> >>   	 * Stack or PC alignment exception handling
> >>
> > What happens if you were running at EL2 when this faults gets injected?
> > It looks like KVM needs something similar, doesn't it?
> >
> > Thanks,
> >
> > 	M.
> Thank you for your comment. I don't think this case is possible, or at 
> least the current KVM code suggests that this case should never happen.  
> In the EL1 code, we get to this case via the vector:
> 
> ventry  el1_sync                        // Synchronous EL1h
> 
> The EL2 KVM equivalent appears to be in arch/arm64/kvm/hyp-entry.S and is:
> 
> ventry  el2h_sync_invalid               // Synchronous EL2h
> 
> This vector is defined as an invalid_vector and has a comment suggesting 
> that it should never happen:
> 
> /* None of these should ever happen */
> ...
>          invalid_vector  el2h_sync_invalid
> 
> Please correct me if I am wrong, but it looks like this case should not 
> be possible.

This comments really means that we shouldn't ever take any of these
exception. If we do, we'll crash and burn (just like the kernel didn't
expect to take an instruction fault from the kernel itself, up until
this patch).

I expect that the firmware does inject the fault into the exception
level it has preempted. So let me turn the question the other way
around: what guarantees that we will never have to handle such a fault
at EL2?

As a corollary, what happens when the firmware injects a fault
triggered by a VM running at EL1, under the control of a hypervisor
running at EL2? There should be some form of exception delegation to
the hypervisor, which makes the lack of handling at EL2 even more
worrying.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny.