On Thu, Nov 02, 2023, Xiaoyao Li wrote: > On 11/2/2023 1:36 AM, Sean Christopherson wrote: > > > KVM_CAP_MEMORY_FAULT_INFO is x86 only, is it better to put this function to > > > <asm/kvm_host.h>? > > I'd prefer to keep it in generic code, as it's highly likely to end up there > > sooner than later. There's a known use case for ARM (exit to userspace on missing > > userspace mapping[*]), and I'm guessing pKVM (also ARM) will also utilize this API. > > > > [*]https://lore.kernel.org/all/20230908222905.1321305-8-amoorthy@xxxxxxxxxx > > I wonder how this CAP is supposed to be checked in userspace, for guest > memfd case? It's basically useless for guest_memfd. > if (!kvm_check_extension(s, KVM_CAP_MEMORY_FAULT_INFO) && > run->exit_reason == KVM_EXIT_MEMORY_FAULT) > abort("unexpected KVM_EXIT_MEMORY_FAULT"); > > In my implementation of QEMU patches, I find it's unnecessary. When > userspace gets an exit with KVM_EXIT_MEMORY_FAULT, it implies > "KVM_CAP_MEMORY_FAULT_INFO". > > So I don't see how it is necessary in this series. Whether it's necessary or > not for [*], I don't have the answer but we can leave the discussion to that > patch series. It's not strictly necessary there either. However, Oliver felt (and presumably still feels) quite strongly, and I agree, that neither reporting extra information shouldn't be tightly coupled to KVM_CAP_EXIT_ON_MISSING or KVM_CAP_GUEST_MEMFD. E.g. if userspace develops a "standalone" use case for KVM_CAP_MEMORY_FAULT_INFO, userspace should be able to check for support without having to take a dependency on KVM_CAP_GUEST_MEMFD, especially since because KVM_CAP_GUEST_MEMFD may not be supported, i.e. userspace should be able to do: if (!kvm_check_extension(s, KVM_CAP_MEMORY_FAULT_INFO)) abort("KVM_CAP_MEMORY_FAULT_INFO required for fancy feature XYZ");
