On Fri, Oct 27, 2023 at 7:23 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > Initialize run->exit_reason to KVM_EXIT_UNKNOWN early in KVM_RUN to reduce > the probability of exiting to userspace with a stale run->exit_reason that > *appears* to be valid. > > To support fd-based guest memory (guest memory without a corresponding > userspace virtual address), KVM will exit to userspace for various memory > related errors, which userspace *may* be able to resolve, instead of using > e.g. BUS_MCEERR_AR. And in the more distant future, KVM will also likely > utilize the same functionality to let userspace "intercept" and handle > memory faults when the userspace mapping is missing, i.e. when fast gup() > fails. > > Because many of KVM's internal APIs related to guest memory use '0' to > indicate "success, continue on" and not "exit to userspace", reporting > memory faults/errors to userspace will set run->exit_reason and > corresponding fields in the run structure fields in conjunction with a > a non-zero, negative return code, e.g. -EFAULT or -EHWPOISON. And because > KVM already returns -EFAULT in many paths, there's a relatively high > probability that KVM could return -EFAULT without setting run->exit_reason, > in which case reporting KVM_EXIT_UNKNOWN is much better than reporting > whatever exit reason happened to be in the run structure. > > Note, KVM must wait until after run->immediate_exit is serviced to > sanitize run->exit_reason as KVM's ABI is that run->exit_reason is > preserved across KVM_RUN when run->immediate_exit is true. > > Link: https://lore.kernel.org/all/20230908222905.1321305-1-amoorthy@xxxxxxxxxx > Link: https://lore.kernel.org/all/ZFFbwOXZ5uI%2Fgdaf@xxxxxxxxxx > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> > --- Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx> Tested-by: Fuad Tabba <tabba@xxxxxxxxxx> Cheers, /fuad > arch/x86/kvm/x86.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index ee3cd8c3c0ef..f41dbb1465a0 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -10963,6 +10963,7 @@ static int vcpu_run(struct kvm_vcpu *vcpu) > { > int r; > > + vcpu->run->exit_reason = KVM_EXIT_UNKNOWN; > vcpu->arch.l1tf_flush_l1d = true; > > for (;;) { > -- > 2.42.0.820.g83a721a137-goog >
