Re: [PATCH 21/27] arm64/sve: KVM: Prevent guests from using SVE

[Dave Martin <Dave.Martin@arm.com>]

On Tue, Aug 15, 2017 at 05:33:15PM +0100, Marc Zyngier wrote:
> On 09/08/17 13:05, Dave Martin wrote:
> > Until KVM has full SVE support, guests must not be allowed to
> > execute SVE instructions.
> > 
> > This patch enables the necessary traps, and also ensures that the
> > traps are disabled again on exit from the guest so that the host
> > can still use SVE if it wants to.
> > 
> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > ---
> >  arch/arm64/include/asm/kvm_arm.h | 3 ++-
> >  arch/arm64/kvm/hyp/switch.c      | 6 +++---
> >  2 files changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> > index dbf0537..8a19651 100644
> > --- a/arch/arm64/include/asm/kvm_arm.h
> > +++ b/arch/arm64/include/asm/kvm_arm.h
> > @@ -186,7 +186,7 @@
> >  #define CPTR_EL2_TTA	(1 << 20)
> >  #define CPTR_EL2_TFP	(1 << CPTR_EL2_TFP_SHIFT)
> >  #define CPTR_EL2_TZ	(1 << 8)
> > -#define CPTR_EL2_DEFAULT	0x000033ff
> > +#define CPTR_EL2_DEFAULT	(0x000033ff & ~CPTR_EL2_TZ)
> 
> I must say I'm not overly fond of this construct. I'd rather introduce a
> RES1 field that matches the v8.2 description, instead of this ugly
> constant and something that clears it.

Sorry, I don't get your meaning here.  v8.2 neither immediately predates
or postdates SVE.  What are you propsing?

I guess we could just write 0x000032ff now that the only meaning the
architecture can ever assign to bit 8 is known.

[...]

Cheers
---Dave