On Thu, Nov 02, 2023, Kai Huang wrote: > On Wed, 2023-11-01 at 10:36 -0700, Sean Christopherson wrote: > > On Wed, Nov 01, 2023, Kai Huang wrote: > > > > > > > +7.34 KVM_CAP_MEMORY_FAULT_INFO > > > > +------------------------------ > > > > + > > > > +:Architectures: x86 > > > > +:Returns: Informational only, -EINVAL on direct KVM_ENABLE_CAP. > > > > + > > > > +The presence of this capability indicates that KVM_RUN will fill > > > > +kvm_run.memory_fault if KVM cannot resolve a guest page fault VM-Exit, e.g. if > > > > +there is a valid memslot but no backing VMA for the corresponding host virtual > > > > +address. > > > > + > > > > +The information in kvm_run.memory_fault is valid if and only if KVM_RUN returns > > > > +an error with errno=EFAULT or errno=EHWPOISON *and* kvm_run.exit_reason is set > > > > +to KVM_EXIT_MEMORY_FAULT. > > > > > > IIUC returning -EFAULT or whatever -errno is sort of KVM internal > > > implementation. > > > > The errno that is returned to userspace is ABI. In KVM, it's a _very_ poorly > > defined ABI for the vast majority of ioctls(), but it's still technically ABI. > > KVM gets away with being cavalier with errno because the vast majority of errors > > are considered fatal by userespace, i.e. in most cases, userspace simply doesn't > > care about the exact errno. > > > > A good example is KVM_RUN with -EINTR; if KVM were to return something other than > > -EINTR on a pending signal or vcpu->run->immediate_exit, userspace would fall over. > > > > > Is it better to relax the validity of kvm_run.memory_fault when > > > KVM_RUN returns any -errno? > > > > Not unless there's a need to do so, and if there is then we can update the > > documentation accordingly. If KVM's ABI is that kvm_run.memory_fault is valid > > for any errno, then KVM would need to purge kvm_run.exit_reason super early in > > KVM_RUN, e.g. to prevent an -EINTR return due to immediate_exit from being > > misinterpreted as KVM_EXIT_MEMORY_FAULT. And purging exit_reason super early is > > subtly tricky because KVM's (again, poorly documented) ABI is that *some* exit > > reasons are preserved across KVM_RUN with vcpu->run->immediate_exit (or with a > > pending signal). > > > > https://lore.kernel.org/all/ZFFbwOXZ5uI%2Fgdaf@xxxxxxxxxx > > > > > > Agreed with not to relax to any errno. However using -EFAULT as part of ABI > definition seems a little bit dangerous, e.g., someone could accidentally or > mistakenly return -EFAULT in KVM_RUN at early time and/or in a completely > different code path, etc. -EINTR has well defined meaning, but -EFAULT (which > is "Bad address") seems doesn't but I am not sure either. :-) KVM has returned -EFAULT since forever, i.e. it's effectively already part of the ABI. I doubt there's a userspace that relies precisely on -EFAULT, but userspace definitely will be confused if KVM returns '0' where KVM used to return -EFAULT. And so if we want to return '0', it needs to be opt-in, which means forcing userspace to enable a capability *and* requires code in KVM to conditionally return '0' instead of -EFAULT/-EHWPOISON. > One example is, for backing VMA with VM_IO | VM_PFNMAP, hva_to_pfn() returns > KVM_PFN_ERR_FAULT when the kernel cannot get a valid PFN (e.g. when SGX vepc > fault handler failed to allocate EPC) and kvm_handle_error_pfn() will just > return -EFAULT. If kvm_run.exit_reason isn't purged early then is it possible > to have some issue here? Well, yeah, but that's exactly why this series has a patch to reset exit_reason. The solution to "if KVM is buggy then bad things happen" is to not have KVM bugs :-)
