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

Commit Message

Christoffer Dall Sept. 3, 2018, 1:45 p.m. UTC

From: Marc Zyngier <[e-mail]>

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.

Fixes: e6b673b741ea ("KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing")
Cc: Dave Martin <[e-mail]>
Reported-by: Alexander Graf <[e-mail]>
Tested-by: Alexander Graf <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Signed-off-by: Christoffer Dall <[e-mail]>
---
 arch/arm64/kvm/hyp/switch.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Comments

Dave Martin Sept. 4, 2018, 11:39 a.m. UTC | #1

On Mon, Sep 03, 2018 at 03:45:14PM +0200, Christoffer Dall wrote:
> From: Marc Zyngier <[e-mail]>
> 
> 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.

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


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 <[e-mail]>
> Reported-by: Alexander Graf <[e-mail]>
> Tested-by: Alexander Graf <[e-mail]>
> Signed-off-by: Marc Zyngier <[e-mail]>
> Signed-off-by: Christoffer Dall <[e-mail]>
> ---
>  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
> 
> 
> _______________________________________________
> linux-arm-kernel mailing list
> [e-mail]
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

Christoffer Dall Sept. 4, 2018, 12:51 p.m. UTC | #2

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 <[e-mail]>
> > 
> > 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 <[e-mail]>
> > Reported-by: Alexander Graf <[e-mail]>
> > Tested-by: Alexander Graf <[e-mail]>
> > Signed-off-by: Marc Zyngier <[e-mail]>
> > Signed-off-by: Christoffer Dall <[e-mail]>
> > ---
> >  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
> > 
> > 
> > _______________________________________________
> > linux-arm-kernel mailing list
> > [e-mail]
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

Dave Martin Sept. 5, 2018, 11:28 a.m. UTC | #3

On Tue, Sep 04, 2018 at 02:51:21PM +0200, Christoffer Dall wrote:
> 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 <[e-mail]>
> > > 
> > > 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.

You can try taking a look at the archietctural pseudocode -- see the
CheckFPAdvSIMDEnabled64() (for AArch64) and CheckFPAdvSIMDEnabled() (for
AArch32) function hierarchies on developer.arm.com.

ARM DDI 0487B.b says for FPEXC32_EL2:

"Purpose:  Allows access to the AArch32 register FPEXC from AArch64
state only. Its value has no effect on execution in AArch64 state."

This could arguably be worded better, but the second sentence is
pretty unequivocal.

> 
> > 
> > The wrinkle this patch fixes is that implicit reads of FPEXC by
> 
> what do you mean by implicit reads here?

In effect any trappable instruction can be thought of as reading a bunch
of system registers in order to decide whether to take a trap or execute
the instruction normally.  The pseudocode adopts this model.  These
reads are not done by actual MRS instructions in the instruction stream,
so they are deemed implicit.

[...]

Cheers
---Dave

Christoffer Dall Sept. 5, 2018, 3:03 p.m. UTC | #4

On Wed, Sep 05, 2018 at 12:28:46PM +0100, Dave Martin wrote:
> On Tue, Sep 04, 2018 at 02:51:21PM +0200, Christoffer Dall wrote:
> > 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 <[e-mail]>
> > > > 
> > > > 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.
> 
> You can try taking a look at the archietctural pseudocode -- see the
> CheckFPAdvSIMDEnabled64() (for AArch64) and CheckFPAdvSIMDEnabled() (for
> AArch32) function hierarchies on developer.arm.com.
> 
> ARM DDI 0487B.b says for FPEXC32_EL2:
> 
> "Purpose:  Allows access to the AArch32 register FPEXC from AArch64
> state only. Its value has no effect on execution in AArch64 state."
> 
> This could arguably be worded better, but the second sentence is
> pretty unequivocal.
> 

I think I was confused about your point.  I thought you were arguing
that CPTR_EL2.TFP==1 + FPEXC_EL2.EN==0, and running in AArch32 EL1+EL0,
and accessing VFP at EL0 would trap to EL2, when in fact it traps to
EL1.  But now I think you're arguing that EL2 is unaffected by
FPEXC32_EL2, and I agree with that.

However, given the above, I don't understand how we can let the guest be
in control of FPEXC, if we want to make sure we trap VFP accesses?

I'll comment on the other patch.

Thanks,
     Christoffer

Dave Martin Sept. 7, 2018, 11:19 a.m. UTC | #5

On Wed, Sep 05, 2018 at 05:03:21PM +0200, Christoffer Dall wrote:
> On Wed, Sep 05, 2018 at 12:28:46PM +0100, Dave Martin wrote:

[...]

> > You can try taking a look at the archietctural pseudocode -- see the
> > CheckFPAdvSIMDEnabled64() (for AArch64) and CheckFPAdvSIMDEnabled() (for
> > AArch32) function hierarchies on developer.arm.com.
> > 
> > ARM DDI 0487B.b says for FPEXC32_EL2:
> > 
> > "Purpose:  Allows access to the AArch32 register FPEXC from AArch64
> > state only. Its value has no effect on execution in AArch64 state."
> > 
> > This could arguably be worded better, but the second sentence is
> > pretty unequivocal.
> > 
> 
> I think I was confused about your point.  I thought you were arguing
> that CPTR_EL2.TFP==1 + FPEXC_EL2.EN==0, and running in AArch32 EL1+EL0,
> and accessing VFP at EL0 would trap to EL2, when in fact it traps to
> EL1.  But now I think you're arguing that EL2 is unaffected by
> FPEXC32_EL2, and I agree with that.
> 
> However, given the above, I don't understand how we can let the guest be
> in control of FPEXC, if we want to make sure we trap VFP accesses?

If the guest set FPEXC.EN to 0, then it is expecting its own VFP
accesses to trap to EL1.  If a VFP access attempt traps to EL1 and the
guest is expecting it, why does hyp need to get involved?

Hyp only needs to interfere when an otherwise untrapped access to the
VFP state would occur.

If we don't have the guest context loaded, we would still trap any
attempt by the guest to modify FPEXC to EL2 as a result of CPTR_EL2.TFP
still being set (or equivalent).

Cheers
---Dave

Patch

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