On Mon, 10 May 2021 15:55:28 +0100, Alexandru Elisei <[email protected]> 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 protected]> > > Signed-off-by: Marc Zyngier <[email protected]> > > Cc: [email protected] # 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. _______________________________________________ kvmarm mailing list [email protected] https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
