Subject: Re: [PATCH 2/2] KVM: arm64: Commit pending PC adjustemnts before returning to userspace

From: Marc Zyngier <[email-masked]>
Date: Mon, 10 May 2021 16:04:53 +0100

On Mon, 10 May 2021 15:55:28 +0100,
Alexandru Elisei <[email-masked]> wrote:
> 
> Hi Marc,
> 
> On 5/10/21 10:49 AM, Marc Zyngier wrote:
> > KVM currently updates PC (and the corresponding exception state)
> > using a two phase approach: first by setting a set of flags,
> > then by converting these flags into a state update when the vcpu
> > is about to enter the guest.
> >
> > However, this creates a disconnect with userspace if the vcpu thread
> > returns there with any exception/PC flag set. In this case, the exposed
> 
> The code seems to handle only the KVM_ARM64_PENDING_EXCEPTION
> flag. Is the "PC flag" a reference to the KVM_ARM64_INCREMENT_PC
> flag?

No, it does handle both exception and PC increment, unless I have
completely bodged something (entirely possible).

> 
> > context is wrong, as userpsace doesn't have access to these flags
> 
> s/userpsace/userspace
> 
> > (they aren't architectural). It also means that these flags are
> > preserved across a reset, which isn't expected.
> >
> > To solve this problem, force an explicit synchronisation of the
> > exception state on vcpu exit to userspace. As an optimisation
> > for nVHE systems, only perform this when there is something pending.
> >
> > Reported-by: Zenghui Yu <[email-masked]>
> > Signed-off-by: Marc Zyngier <[email-masked]>
> > Cc: [email-masked] # 5.11
> > ---
> >  arch/arm64/include/asm/kvm_asm.h   |  1 +
> >  arch/arm64/kvm/arm.c               | 10 ++++++++++
> >  arch/arm64/kvm/hyp/exception.c     |  4 ++--
> >  arch/arm64/kvm/hyp/nvhe/hyp-main.c |  8 ++++++++
> >  4 files changed, 21 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> > index d5b11037401d..5e9b33cbac51 100644
> > --- a/arch/arm64/include/asm/kvm_asm.h
> > +++ b/arch/arm64/include/asm/kvm_asm.h
> > @@ -63,6 +63,7 @@
> >  #define __KVM_HOST_SMCCC_FUNC___pkvm_cpu_set_vector		18
> >  #define __KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize		19
> >  #define __KVM_HOST_SMCCC_FUNC___pkvm_mark_hyp			20
> > +#define __KVM_HOST_SMCCC_FUNC___kvm_adjust_pc			21
> >  
> >  #ifndef __ASSEMBLY__
> >  
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 1cb39c0803a4..d62a7041ebd1 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -897,6 +897,16 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> >  
> >  	kvm_sigset_deactivate(vcpu);
> >  
> > +	/*
> > +	 * In the unlikely event that we are returning to userspace
> > +	 * with pending exceptions or PC adjustment, commit these
> 
> I'm going to assume "PC adjustment" means the KVM_ARM64_INCREMENT_PC
> flag. Please correct me if that's not true, but if that's the case,
> then the flag isn't handled below.
> 
> > +	 * adjustments in order to give userspace a consistent view of
> > +	 * the vcpu state.
> > +	 */
> > +	if (unlikely(vcpu->arch.flags & (KVM_ARM64_PENDING_EXCEPTION |
> > +					 KVM_ARM64_EXCEPT_MASK)))
> 
> The condition seems to suggest that it is valid to set
> KVM_ARM64_EXCEPT_{AA32,AA64}_* without setting
> KVM_ARM64_PENDING_EXCEPTION, which looks rather odd to me.
> Is that a valid use of the KVM_ARM64_EXCEPT_MASK bits? If it's not
> (the existing code always sets the exception type with the
> KVM_ARM64_PENDING_EXCEPTION), that I was thinking that checking only
> the KVM_ARM64_PENDING_EXCEPTION flag would make the intention
> clearer.

No, you are missing this (subtle) comment in kvm_host.h:

<quote>
/*
 * Overlaps with KVM_ARM64_EXCEPT_MASK on purpose so that it can't be
 * set together with an exception...
 */
#define KVM_ARM64_INCREMENT_PC		(1 << 9) /* Increment PC */
</quote>

So (KVM_ARM64_PENDING_EXCEPTION | KVM_ARM64_EXCEPT_MASK) checks for
*both* an exception and a PC increment.

Thanks,

	M.

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

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