Subject: Re: [PATCH 2/5] arm64: Handle UNDEF in the EL2 stub vectors

From: Marc Zyngier <[email-masked]>
Date: Fri, 13 Aug 2021 18:41:36 +0100

On Fri, 13 Aug 2021 14:08:23 +0100,
Robin Murphy <[email-masked]> wrote:
> 
> On 2021-08-12 20:02, Marc Zyngier wrote:
> > As we want to handle the silly case where HVC has been disabled
> > from EL3, augment our ability to handle exception at EL2.
> > 
> > Check for unknown exceptions (usually UNDEF) coming from EL2,
> > and treat them as a failing HVC call into the stubs. While
> > this isn't great and obviously doesn't catter for the gigantic
> > range of possible exceptions, it isn't any worse than what we
> > have today.
> > 
> > Just don't try and use it for anything else.
> > 
> > Signed-off-by: Marc Zyngier <[email-masked]>
> > ---
> >   arch/arm64/kernel/hyp-stub.S | 19 ++++++++++++++++++-
> >   1 file changed, 18 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kernel/hyp-stub.S b/arch/arm64/kernel/hyp-stub.S
> > index 43d212618834..026a34515b21 100644
> > --- a/arch/arm64/kernel/hyp-stub.S
> > +++ b/arch/arm64/kernel/hyp-stub.S
> > @@ -46,7 +46,16 @@ SYM_CODE_END(__hyp_stub_vectors)
> >   	.align 11
> >     SYM_CODE_START_LOCAL(elx_sync)
> > -	cmp	x0, #HVC_SET_VECTORS
> > +	mrs	x4, spsr_el2
> > +	and	x4, x4, #PSR_MODE_MASK
> > +	orr	x4, x4, #1
> > +	cmp	x4, #PSR_MODE_EL2h
> > +	b.ne	0f
> > +	mrs	x4, esr_el2
> > +	eor	x4, x4, #ESR_ELx_IL
> > +	cbz	x4, el2_undef
> 
> Hmm, might it be neater to check ESR_EL2.ISS to see if we landed here
> for any reason *other* than a successfully-executed HVC?

We absolutely could. However, the sixpence question (yes, that's the
Brexit effect for you) is "what do you do with exceptions that are
neither UNDEF now HVC?".

We are taking a leap of faith by assuming that the only thing that
will UNDEF at EL2 while the stubs are installed is HVC. If anything
else occurs, I have no idea what to do with it. I guess we could always
ignore it instead of treating it as a HVC (as it is done at the
moment).

Thanks,

	M.

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

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