Hi Suzuki, On Tue, 09 Mar 2021 11:09:48 +0000, Suzuki Poulose <suzuki.poulose@xxxxxxx> wrote: > > > > > On 8 Mar 2021, at 17:46, Marc Zyngier <maz@xxxxxxxxxx> wrote: > > > > KVM/arm64 has forever used a 40bit default IPA space, partially > > due to its 32bit heritage (where the only choice is 40bit). > > > > However, there are implementations in the wild that have a *cough* > > much smaller *cough* IPA space, which leads to a misprogramming of > > VTCR_EL2, and a guest that is stuck on its first memory access > > if userspace dares to ask for the default IPA setting (which most > > VMMs do). > > > > Instead, cap the default IPA size to what the host can actually > > do, and spit out a one-off message on the console. The boot warning > > is turned into a more meaningfull message, and the new behaviour > > is also documented. > > > > Although this is a userspace ABI change, it doesn't really change > > much for userspace: > > > > - the guest couldn't run before this change, while it now has > > a chance to if the memory range fits the reduced IPA space > > > > - a memory slot that was accepted because it did fit the default > > IPA space but didn't fit the HW constraints is now properly > > rejected > > > > The other thing that's left doing is to convince userspace to > > actually use the IPA space setting instead of relying on the > > antiquated default. > > > > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx> > > --- > > Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx> Thanks for that. Whilst I have your attention and given that you are responsible for most of the variable IPA stuff... ;-) I think we have another issue around the handling of our IPA size. Let's say I create a VM with a 32bit IPA space. If I register a 2GB memslot at 0x8000000, I'm getting an error, which I think is bogus. I came to the conclusion that kvm_arch_prepare_memory_region() is a bit overzealous when rejecting the memslot, and I used the following patchlet to address it. Does this seem sensible to you? M. diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c index 77cb2d28f2a4..8711894db8c2 100644 --- a/arch/arm64/kvm/mmu.c +++ b/arch/arm64/kvm/mmu.c @@ -1312,8 +1312,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm, * Prevent userspace from creating a memory region outside of the IPA * space addressable by the KVM guest IPA space. */ - if (memslot->base_gfn + memslot->npages >= - (kvm_phys_size(kvm) >> PAGE_SHIFT)) + if ((memslot->base_gfn + memslot->npages) > (kvm_phys_size(kvm) >> PAGE_SHIFT)) return -EFAULT; mmap_read_lock(current->mm); -- Without deviation from the norm, progress is not possible.
