Subject: Re: [PATCH 3/5] arm64: Detect disabled HVC early

From: Marc Zyngier <[email-masked]>
Date: Fri, 13 Aug 2021 18:33:58 +0100

On Fri, 13 Aug 2021 10:05:40 +0100,
Will Deacon <[email-masked]> wrote:
> 
> On Thu, Aug 12, 2021 at 08:02:11PM +0100, Marc Zyngier wrote:
> > Having HVC disabled from EL3 while the kernel is entered at EL2
> > is a complete nightmare.
> > 
> > We end-up taking an UNDEF at the worse possible moment (checking
> > for VHE) and even if we didn't, having KVM enabled would signify
> > the premature end of the kernel.
> > 
> > Instead, try and detect this stupid case by issuing a HVC
> > for HVC_RESET_VECTORS, which does nothing when the stubs
> > are live. If we get HVC_STUB_ERR back, that's because the
> > UNDEF handler has kicked in.
> > 
> > In this situation, close your eyes, block your nose, and gracefully
> > pretend we have booted at EL1.
> > 
> > Reported-by: Rafał Miłecki <[email-masked]>
> > Signed-off-by: Marc Zyngier <[email-masked]>
> > ---
> >  arch/arm64/kernel/head.S | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> > 
> > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > index 9d5aa56a98cc..d6b2b05f5d3a 100644
> > --- a/arch/arm64/kernel/head.S
> > +++ b/arch/arm64/kernel/head.S
> > @@ -519,6 +519,28 @@ SYM_INNER_LABEL(init_el2, SYM_L_LOCAL)
> >  	msr	vbar_el2, x0
> >  	isb
> >  
> > +	// Check that HVC actually works...
> > +	mov	x0, #HVC_RESET_VECTORS
> > +	hvc	#0
> > +
> > +	mov_q	x1, HVC_STUB_ERR
> > +	cmp	x0, x1
> > +	b.ne	0f
> > +
> > +	/*
> > +	 * HVC is unusable, so pretend we actually booted at EL1.
> > +	 * Once we have left EL2, there will be no going back.
> > +	 */
> 
> This comment got me thinking...
> 
> 	.macro	host_hvc0
> 	mrs	xzr, actlr_el1
> 	.endm
> 
> then set HCR_EL2.TACR=1 while we still can and match the ISS against a
> constant in the handler. Too awful?

I had a similar idea, using an ID register instead (though ACTLR_EL1
is much neater). It would indeed allow the kernel to go back to EL2.

But for what purpose? Yes, we could enable KVM that way. However, the
guest would need to know about it as well, as even simple things like
PSCI wouldn't work. That's would be defining a new guest ABI, and I'm
not overly keen on it.

Unless you are thinking of something else?

Thanks,

	M.

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

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