Subject: Re: [PATCH v2] KVM: arm64: Don't set HCR_EL2.TVM when S2FWB is supported

From: Marc Zyngier <[email-masked]>
Date: Mon, 28 Oct 2019 16:19:55 +0000

On Mon, 28 Oct 2019 15:12:39 +0000,
Alexandru Elisei <[email-masked]> wrote:
> 
> Hi,
> 
> Resending this email, because I replied only to the kvmarm list by accident,
> instead of replying to everyone involved.
> 
> On 10/28/19 1:05 PM, Christoffer Dall wrote:
> > On CPUs that support S2FWB (Armv8.4+), KVM configures the stage 2 page
> > tables to override the memory attributes of memory accesses, regardless
> > of the stage 1 page table configurations, and also when the stage 1 MMU
> > is turned off.  This results in all memory accesses to RAM being
> > cacheable, including during early boot of the guest.
> >
> > On CPUs without this feature, memory accesses were non-cacheable during
> > boot until the guest turned on the stage 1 MMU, and we had to detect
> > when the guest turned on the MMU, such that we could invalidate all cache
> > entries and ensure a consistent view of memory with the MMU turned on.
> > When the guest turned on the caches, we would call stage2_flush_vm()
> > from kvm_toggle_cache().
> >
> > However, stage2_flush_vm() walks all the stage 2 tables, and calls
> > __kvm_flush-dcache_pte, which on a system with S2FWD does ... absolutely
> > nothing.
> >
> > We can avoid that whole song and dance, and simply not set TVM when
> > creating a VM on a system that has S2FWB.
> >
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > Reviewed-by: Mark Rutland <[email-masked]>
> > ---
> > I was only able to test this on the model with cache modeling enabled,
> > but even removing TVM from HCR_EL2 without having FWB also worked with
> > that setup, so the testing of this has been light.  It seems like it
> > should obviously work, but it would be good if someone with access to
> > appropriate hardware could give this a spin.
> >
> >  arch/arm64/include/asm/kvm_arm.h     |  3 +--
> >  arch/arm64/include/asm/kvm_emulate.h | 12 +++++++++++-
> >  2 files changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> > index ddf9d762ac62..6e5d839f42b5 100644
> > --- a/arch/arm64/include/asm/kvm_arm.h
> > +++ b/arch/arm64/include/asm/kvm_arm.h
> > @@ -61,7 +61,6 @@
> >   * RW:		64bit by default, can be overridden for 32bit VMs
> >   * TAC:		Trap ACTLR
> >   * TSC:		Trap SMC
> > - * TVM:		Trap VM ops (until M+C set in SCTLR_EL1)
> >   * TSW:		Trap cache operations by set/way
> >   * TWE:		Trap WFE
> >   * TWI:		Trap WFI
> > @@ -74,7 +73,7 @@
> >   * SWIO:	Turn set/way invalidates into set/way clean+invalidate
> >   */
> >  #define HCR_GUEST_FLAGS (HCR_TSC | HCR_TSW | HCR_TWE | HCR_TWI | HCR_VM | \
> > -			 HCR_TVM | HCR_BSU_IS | HCR_FB | HCR_TAC | \
> > +			 HCR_BSU_IS | HCR_FB | HCR_TAC | \
> >  			 HCR_AMO | HCR_SWIO | HCR_TIDCP | HCR_RW | HCR_TLOR | \
> >  			 HCR_FMO | HCR_IMO)
> >  #define HCR_VIRT_EXCP_MASK (HCR_VSE | HCR_VI | HCR_VF)
> > diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> > index d69c1efc63e7..70509799a2a9 100644
> > --- a/arch/arm64/include/asm/kvm_emulate.h
> > +++ b/arch/arm64/include/asm/kvm_emulate.h
> > @@ -53,8 +53,18 @@ static inline void vcpu_reset_hcr(struct kvm_vcpu *vcpu)
> >  		/* trap error record accesses */
> >  		vcpu->arch.hcr_el2 |= HCR_TERR;
> >  	}
> > -	if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB))
> > +
> > +	if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB)) {
> >  		vcpu->arch.hcr_el2 |= HCR_FWB;
> > +	} else {
> > +		/*
> > +		 * For non-FWB CPUs, we trap VM ops (HCR_EL2.TVM) until M+C
> > +		 * get set in SCTLR_EL1 such that we can detect when the guest
> > +		 * MMU gets turned off and do the necessary cache maintenance
> > +		 * then.
> > +		 */
> > +		vcpu->arch.hcr_el2 &= ~HCR_TVM;
> 
> Don't we want to set the bit here, so we're consistent with the
> previous behaviour and the comment? Because with this patch, we
> never set HCR_EL2.TVM...

Of course you're right. This is how I plan to fix it:

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index 47c774c2d18b..7b835337f78b 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -63,7 +63,7 @@ static inline void vcpu_reset_hcr(struct kvm_vcpu *vcpu)
 		 * MMU gets turned on and do the necessary cache maintenance
 		 * then.
 		 */
-		vcpu->arch.hcr_el2 &= ~HCR_TVM;
+		vcpu->arch.hcr_el2 |= HCR_TVM;
 	}
 
 	if (test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features))

Christoffer, please shout if you disagree.

	M.

-- 
Jazz is not dead, it just smells funny.

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