Re: [PATCH 15/18] KVM: arm64: Warn when PENDING_EXCEPTION and INCREMENT_PC are set together

From: Marc Zyngier <maz@kernel.org>
To: Fuad Tabba <tabba@google.com>
Cc: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Oliver Upton <oupton@google.com>, Will Deacon <will@kernel.org>,
	Quentin Perret <qperret@google.com>,
	Mark Brown <broonie@kernel.org>,
	kernel-team@android.com
Subject: Re: [PATCH 15/18] KVM: arm64: Warn when PENDING_EXCEPTION and INCREMENT_PC are set together
Date: Wed, 08 Jun 2022 17:42:11 +0100	[thread overview]
Message-ID: <87h74v3y58.wl-maz@kernel.org> (raw)
In-Reply-To: <CA+EHjTyW62HaJdH_L1RMBzQpzkNbFBAYpXQ-y_Wg4u1a2eVJwA@mail.gmail.com>

On Wed, 08 Jun 2022 16:16:55 +0100,
Fuad Tabba <tabba@google.com> wrote:
> 
> Hi Marc,
> 
> On Sat, May 28, 2022 at 12:49 PM Marc Zyngier <maz@kernel.org> wrote:
> >
> > We really don't want PENDING_EXCEPTION and INCREMENT_PC to ever be
> > set at the same time, as they are mutually exclusive. Add checks
> > that will generate a warning should this ever happen.
> >
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/include/asm/kvm_emulate.h | 1 +
> >  arch/arm64/kvm/hyp/nvhe/sys_regs.c   | 2 ++
> >  arch/arm64/kvm/inject_fault.c        | 8 ++++++++
> >  3 files changed, 11 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> > index 46e631cd8d9e..861fa0b24a7f 100644
> > --- a/arch/arm64/include/asm/kvm_emulate.h
> > +++ b/arch/arm64/include/asm/kvm_emulate.h
> > @@ -473,6 +473,7 @@ static inline unsigned long vcpu_data_host_to_guest(struct kvm_vcpu *vcpu,
> >
> >  static __always_inline void kvm_incr_pc(struct kvm_vcpu *vcpu)
> >  {
> > +       WARN_ON(vcpu_get_flag(vcpu, PENDING_EXCEPTION));
> >         vcpu_set_flag(vcpu, INCREMENT_PC);
> >  }
> >
> > diff --git a/arch/arm64/kvm/hyp/nvhe/sys_regs.c b/arch/arm64/kvm/hyp/nvhe/sys_regs.c
> > index 2841a2d447a1..04973984b6db 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/sys_regs.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/sys_regs.c
> > @@ -38,6 +38,8 @@ static void inject_undef64(struct kvm_vcpu *vcpu)
> >         *vcpu_pc(vcpu) = read_sysreg_el2(SYS_ELR);
> >         *vcpu_cpsr(vcpu) = read_sysreg_el2(SYS_SPSR);
> >
> > +       WARN_ON(vcpu_get_flag(vcpu, INCREMENT_PC));
> > +
> >         vcpu_set_flag(vcpu, PENDING_EXCEPTION);
> >         vcpu_set_flag(vcpu, EXCEPT_AA64_EL1_SYNC);
> >
> > diff --git a/arch/arm64/kvm/inject_fault.c b/arch/arm64/kvm/inject_fault.c
> > index a9a7b513f3b0..2f4b9afc16ec 100644
> > --- a/arch/arm64/kvm/inject_fault.c
> > +++ b/arch/arm64/kvm/inject_fault.c
> > @@ -20,6 +20,8 @@ static void inject_abt64(struct kvm_vcpu *vcpu, bool is_iabt, unsigned long addr
> >         bool is_aarch32 = vcpu_mode_is_32bit(vcpu);
> >         u32 esr = 0;
> >
> > +       WARN_ON(vcpu_get_flag(vcpu, INCREMENT_PC));
> > +
> 
> Minor nit: While we're at it, should we just create a helper for
> setting PENDING_EXCEPTION, same as we have for INCREMENT_PC? That
> might make the code clearer and save us from the hassle of having this
> WARN_ON before every instance of setting PENDING_EXCEPTION?

Good point. I ended up with this:

#define kvm_pend_exception(v, e)					\
	do {								\
		WARN_ON(vcpu_get_flag((v), INCREMENT_PC));		\
		vcpu_set_flag((v), PENDING_EXCEPTION);			\
		vcpu_set_flag((v), e);					\
	} while (0)

It has to be a macro in order to deal with the flag expansion, but is
otherwise a welcome cleanup.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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