[PULL 2/4] arm64: KVM: Only force FPEXC32_EL2.EN if trapping FPSIMD

[christoffer.dall@arm.com (Christoffer Dall)]

On Tue, Sep 04, 2018 at 12:39:54PM +0100, Dave Martin wrote:
> On Mon, Sep 03, 2018 at 03:45:14PM +0200, Christoffer Dall wrote:
> > From: Marc Zyngier <marc.zyngier@arm.com>
> > 
> > If trapping FPSIMD in the context of an AArch32 guest, it is critical
> > to set FPEXC32_EL2.EN to 1 so that the trapping is taken to EL2 and
> > not EL1.
> > 
> > Conversely, it is just as critical *not* to set FPEXC32_EL2.EN to 1
> > if we're not going to trap FPSIMD, as we then corrupt the existing
> > VFP state.
> > 
> > Moving the call to __activate_traps_fpsimd32 to the point where we
> > know for sure that we are going to trap ensures that we don't set that
> > bit spuriously.
> 
> 
> Hmmm, this looks like a viable fix but actually the way the code is
> structured is a bit confusing here.
> 
> It looks like some unnecessary functionality has been inherited from
> arch/arm here: for a 32-bit host, FPEXC controls FPSIMD trapping even
> at Hyp, so we need to handle it specially in order for the Hyp FPSIMD
> context switching code to run without generating additional traps.
> 
> For a 64-bit host, this doesn't apply: FPEXC32_EL2 is just guest
> context and doesn't affect EL2 execution.

Hmm, is that really true?  Earlier versions of the Armv8 Arm ARM, for
the description of CPTR_EL2.TFP:

  This causes instructions that access the registers associated with
  Floating Point and Advanced SIMD execution to trap to EL2 when
  executed from EL0, EL1, or EL2, unless trapped to EL1.

In the current version I think you can arrive at the same conclusion by
looking at the FPEXC_EL2.EN description which tells you that if EN==0,
then the instruction is UNDEFINED, and the Synchronous exception
prioritization rules specify that undefined instructions are taken
before trapped intructions because of CPTR_EL2.

But there does seem to be some inconsistency with the description of
generally trapping floating point access to EL2.

> 
> The wrinkle this patch fixes is that implicit reads of FPEXC by

what do you mean by implicit reads here?

> the guest are in effect not trappable, so if this register is wrongly
> configured for the guest, spurious traps to EL1 may occur that are
> not architecturally expected by the guest, which is precisely the
> problem that Alexander hit.  I missed this issue in my previous
> refactoring...
> 

Thanks,

    Christoffer

> 
> So, maybe FPEXC32_EL2 should just be treated as another guest system
> register that must be eagerly switched.
> 
> This would also avoid the double write of FPEXC32_EL2 that we currently
> do (once when enabling traps, then again when writing the correct guest
> value after a trap is taken), and the isb() can be removed too since
> host FPSIMD accesses won't trap on arm64 irrespective of FPEXC32_EL2.
> 
> 
> I'll try to cook up a cleanup patch to apply on top, but it can be
> treated as non-urgent.
> 
> Cheers
> ---Dave
> 
> > 
> > Fixes: e6b673b741ea ("KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing")
> > Cc: Dave Martin <dave.martin@arm.com>
> > Reported-by: Alexander Graf <agraf@suse.de>
> > Tested-by: Alexander Graf <agraf@suse.de>
> > Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> > Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
> > ---
> >  arch/arm64/kvm/hyp/switch.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > index d496ef579859..ca46153d7915 100644
> > --- a/arch/arm64/kvm/hyp/switch.c
> > +++ b/arch/arm64/kvm/hyp/switch.c
> > @@ -98,8 +98,10 @@ static void activate_traps_vhe(struct kvm_vcpu *vcpu)
> >  	val = read_sysreg(cpacr_el1);
> >  	val |= CPACR_EL1_TTA;
> >  	val &= ~CPACR_EL1_ZEN;
> > -	if (!update_fp_enabled(vcpu))
> > +	if (!update_fp_enabled(vcpu)) {
> >  		val &= ~CPACR_EL1_FPEN;
> > +		__activate_traps_fpsimd32(vcpu);
> > +	}
> >  
> >  	write_sysreg(val, cpacr_el1);
> >  
> > @@ -114,8 +116,10 @@ static void __hyp_text __activate_traps_nvhe(struct kvm_vcpu *vcpu)
> >  
> >  	val = CPTR_EL2_DEFAULT;
> >  	val |= CPTR_EL2_TTA | CPTR_EL2_TZ;
> > -	if (!update_fp_enabled(vcpu))
> > +	if (!update_fp_enabled(vcpu)) {
> >  		val |= CPTR_EL2_TFP;
> > +		__activate_traps_fpsimd32(vcpu);
> > +	}
> >  
> >  	write_sysreg(val, cptr_el2);
> >  }
> > @@ -129,7 +133,6 @@ static void __hyp_text __activate_traps(struct kvm_vcpu *vcpu)
> >  	if (cpus_have_const_cap(ARM64_HAS_RAS_EXTN) && (hcr & HCR_VSE))
> >  		write_sysreg_s(vcpu->arch.vsesr_el2, SYS_VSESR_EL2);
> >  
> > -	__activate_traps_fpsimd32(vcpu);
> >  	if (has_vhe())
> >  		activate_traps_vhe(vcpu);
> >  	else
> > -- 
> > 2.18.0
> > 
> > 
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