On Mon, 10 May 2021 15:55:28 +0100,
Alexandru Elisei <alexandru.elisei@xxxxxxx> 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 <yuzenghui@xxxxxxxxxx>
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # 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.
