Subject: [PATCH] arm: KVM: force execution of HCPTR access on VM exit

From: [email-masked] (Christoffer Dall)
Date: Thu, 9 Apr 2015 15:35:40 +0200

On Wed, Apr 08, 2015 at 06:16:50PM +0100, Marc Zyngier wrote:
> On 08/04/15 12:40, Christoffer Dall wrote:
> > On Mon, Mar 16, 2015 at 10:59:43AM +0000, Marc Zyngier wrote:
> >> On VM entry, we disable access to the VFP registers in order to
> >> perform a lazy save/restore of these registers.
> >>
> >> On VM exit, we restore access, test if we did enable them before,
> >> and save/restore the guest/host registers if necessary. In this
> >> sequence, the FPEXC register is always accessed, irrespective
> >> of the trapping configuration.
> >>
> >> If the guest didn't touch the VFP registers, then the HCPTR access
> >> has now enabled such access, but we're missing a barrier to ensure
> >> architectural execution of the new HCPTR configuration. If the HCPTR
> >> access has been delayed/reordered, the subsequent access to FPEXC
> >> will cause a trap, which we aren't prepared to handle at all.
> >>
> >> The fix is to introduce a barrier that only takes place if the
> >> guest hasn't accessed its view of the VFP registers, making
> >> the access to FPEXC safe.
> >>
> >> Signed-off-by: Marc Zyngier <[email-masked]>
> >> ---
> >>  arch/arm/kvm/interrupts.S | 7 +++++--
> >>  1 file changed, 5 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/arch/arm/kvm/interrupts.S b/arch/arm/kvm/interrupts.S
> >> index 79caf79..3ac7aca 100644
> >> --- a/arch/arm/kvm/interrupts.S
> >> +++ b/arch/arm/kvm/interrupts.S
> >> @@ -175,10 +175,13 @@ __kvm_vcpu_return:
> >>  #ifdef CONFIG_VFPv3
> >>  	@ Save floating point registers we if let guest use them.
> >>  	tst	r2, #(HCPTR_TCP(10) | HCPTR_TCP(11))
> >> -	bne	after_vfp_restore
> >> +	beq	1f
> >> +
> >> +	isb	@ Force execution of HCPTR if we've just reenabled VFP access
> >> +	b	after_vfp_restore
> >>  
> >>  	@ Switch VFP/NEON hardware state to the host's
> >> -	add	r7, vcpu, #VCPU_VFP_GUEST
> >> +1:	add	r7, vcpu, #VCPU_VFP_GUEST
> >>  	store_vfp_state r7
> >>  	add	r7, vcpu, #VCPU_VFP_HOST
> >>  	ldr	r7, [r7]
> >> -- 
> >> 2.1.4
> >>
> > 
> > Reviewed-by: Christoffer Dall <[email-masked]>
> 
> Thanks (I must admit I completely forgot the existence of this patch...).
> 
> I guess we can queue that for -rc1, with a Cc to stable...
> 
yes, I tested it on TC2 (both A15 and A7) and it works like a charm.

-Christoffer