Re: [PATCH 5/6] KVM: arm64: Treat 32bit ID registers as RAZ/WI on 64bit-only system

[Oliver Upton <oliver.upton@linux.dev>]

Hey Marc,

Thanks for the review!

On Tue, Aug 23, 2022 at 06:05:28PM +0100, Marc Zyngier wrote:
> On Wed, 17 Aug 2022 22:48:17 +0100,
> Oliver Upton <oliver.upton@linux.dev> wrote:
> > 
> > One of the oddities of the architecture is that the AArch64 views of the
> > AArch32 ID registers are UNKNOWN if AArch32 isn't implemented at any EL.
> > Nonetheless, KVM exposes these registers to userspace for the sake of
> > save/restore. It is possible that the UNKNOWN value could differ between
> > systems, leading to a rejected write from userspace.
> > 
> > Avoid the issue altogether by handling the AArch32 ID registers as
> > RAZ/WI when on an AArch64-only system.
> > 
> > Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> > ---
> >  arch/arm64/kvm/sys_regs.c | 63 ++++++++++++++++++++++++++-------------
> >  1 file changed, 43 insertions(+), 20 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 9f06c85f26b8..5f6a633182c8 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -1145,6 +1145,20 @@ static unsigned int id_visibility(const struct kvm_vcpu *vcpu,
> >  	return 0;
> >  }
> >  
> > +static unsigned int aa32_id_visibility(const struct kvm_vcpu *vcpu,
> > +				       const struct sys_reg_desc *r)
> > +{
> > +	/*
> > +	 * AArch32 ID registers are UNKNOWN if AArch32 isn't implemented at any
> > +	 * EL. Promote to RAZ/WI in order to guarantee consistency between
> > +	 * systems.
> > +	 */
> > +	if (!kvm_supports_32bit_el0())
> > +		return REG_RAZ | REG_USER_WI;
> 
> This is probably only a nit, but why does one visibility has a _USER_
> tag while the other doesn't? In other word, what sysregs are WI from
> userspace that aren't so from the guest?
> 
> Also, do we have any cases where RAZ and WI would be used
> independently? My gut feeling is that RAZ implies WI in most (all?)
> cases. If this assumption holds, shouldn't we simply rename REG_RAZ to
> REG_RAZ_WI and be done with it?

Yeah, this reads a bit strange, but there is some reason around it (I
think!)

As it applies to ID registers, REG_RAZ already implies RAZ w/ immutable
writes (-EINVAL if something different is written). As such I didn't want
to change the meaning of the other ID registers to WI and only ignore
writes for the registers that could have an UNKNOWN value. Furthermore,
I added the _USER_ tag to make it clear that we aren't magically allowing
writes from the guest to these registers.

I think we will need an additional visibility bit (or special accessor,
which I tried to avoid) to precisely apply WI to the 32bit registers,
but if the _USER_ tag is distracting I can get rid of it. After all,
hardware should politely UNDEF the guest when writing to such a
register.

Thoughts?

--
Thanks,
Oliver
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