On Thu, Oct 5, 2023 at 3:46 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Thu, Oct 05, 2023, Anish Moorthy wrote: > > On Tue, Oct 3, 2023 at 4:46 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > > > The only way a KVM_EXIT_MEMORY_FAULT that actually reaches userspace could be > > > "unreliable" is if something other than a memory_fault exit clobbered the union, > > > but didn't signal its KVM_EXIT_* reason. And that would be an egregious bug that > > > isn't unique to KVM_EXIT_MEMORY_FAULT, i.e. the same data corruption would affect > > > each and every other KVM_EXIT_* reason. > > > > Keep in mind the case where an "unreliable" annotation sets up a > > KVM_EXIT_MEMORY_FAULT, KVM_RUN ends up continuing, then something > > unrelated comes up and causes KVM_RUN to EFAULT. Although this at > > least is a case of "outdated" information rather than blatant > > corruption. > > Drat, I managed to forget about that. > > > IIRC the last time this came up we said that there's minimal harm in > > userspace acting on the outdated info, but it seems like another good > > argument for just restricting the annotations to paths we know are > > reliable. What if the second EFAULT above is fatal (as I understand > > all are today) and sets up subsequent KVM_RUNs to crash and burn > > somehow? Seems like that'd be a safety issue. > > For your series, let's omit > > KVM: Annotate -EFAULTs from kvm_vcpu_read/write_guest_page > > and just fill memory_fault for the page fault paths. That will be easier to > document too since we can simply say that if the exit reason is KVM_EXIT_MEMORY_FAULT, > then run->memory_fault is valid and fresh. +1 And from a performance perspective, I don't think we care about kvm_vcpu_read/write_guest_page(). Our (Google) KVM Demand Paging implementation just sends any kvm_vcpu_read/write_guest_page() requests through the netlink socket, which is just a poor man's userfaultfd. So I think we'll be fine sending these callsites through uffd instead of exiting out to userspace. And with that out of the way, is there any reason to keep tying KVM_EXIT_MEMORY_FAULT to -EFAULT? As mentioned in the patch at the top of this thread, -EFAULT is just a hack to allow the emulator paths to return out to userspace. But that's no longer necessary. I just find it odd that some KVM_EXIT_* correspond with KVM_RUN returning an error and others don't. The exit_reason is sufficient to tell userspace what's going on and has a firm contract, unlike -EFAULT which anything KVM calls into can return. > > Adding a flag or whatever to mark the data as trustworthy would be the alternative, > but that's effectively adding ABI that says "KVM is buggy, sorry". > > My dream of having KVM always return useful information for -EFAULT will have to > wait for another day.
