On Wed, Jan 10, 2024 at 08:26:25AM -0800, Sean Christopherson wrote: > On Wed, Jan 10, 2024, Chao Gao wrote: > > On Tue, Jan 09, 2024 at 04:23:40PM -0800, Sean Christopherson wrote: > > >Add a VMX flag in /proc/cpuinfo, ept_5level, so that userspace can query > > >whether or not the CPU supports 5-level EPT paging. EPT capabilities are > > >enumerated via MSR, i.e. aren't accessible to userspace without help from > > >the kernel, and knowing whether or not 5-level EPT is supported is sadly > > >necessary for userspace to correctly configure KVM VMs. > > > > This assumes procfs is enabled in Kconfig and userspace has permission to > > access /proc/cpuinfo. But it isn't always true. So, I think it is better to > > advertise max addressable GPA via KVM ioctls. > > Hrm, so the help for PROC_FS says: > > Several programs depend on this, so everyone should say Y here. > > Given that this is working around something that is borderline an erratum, I'm > inclined to say that userspace shouldn't simply assume the worst if /proc isn't > available. Practically speaking, I don't think a "real" VM is likely to be > affected; AFAIK, there's no reason for QEMU or any other VMM to _need_ to expose > a memslot at GPA[51:48] unless the VM really has however much memory that is > (hundreds of terabytes?). And a if someone is trying to run such a massive VM on > such a goofy CPU... It is unusual to assign a huge RAM to guest, but passthrough a device also may trigger this issue which we have met, i.e. alloc memslot for the 64bit BAR which can set bits[51:48]. BIOS can control the BAR address, e.g. seabios moved 64bit pci window to end of address space by using advertised physical bits[1]. [1] https://gitlab.com/qemu-project/seabios/-/commit/bcfed7e270776ab5595cafc6f1794bea0cae1c6c > > I don't think it's unreasonable for KVM selftests to require access to > /proc/cpuinfo. Or actually, they can probably do the same thing and self-limit > to 48-bit addresses if /proc/cpuinfo isn't available. > > I'm not totally opposed to adding a more programmatic way for userspace to query > 5-level EPT support, it just seems unnecessary. E.g. unlike CPUID, userspace > can't directly influence whether or not KVM uses 5-level EPT. Even in hindsight, > I'm not entirely sure KVM should expose such a knob, as it raises questions around > interactions guest.MAXPHYADDR and memslots that I would rather avoid. > > And even if we do add such uAPI, enumerating 5-level EPT in /proc/cpuinfo is > definitely worthwhile, the only thing that would need to be tweaked is the > justification in the changelog. > > One thing we can do irrespective of feature enumeration is have kvm_mmu_page_fault() > exit to userspace with an explicit error if the guest faults ona GPA that KVM > knows it can't map, i.e. exit with KVM_EXIT_INTERNAL_ERROR or maybe even > KVM_EXIT_MEMORY_FAULT instead of looping indefinitely. If KVM does report guest.MAXPHYADDR=host.MAXPHYADDR, it is not reasonable to kill the guest directly. And just reporting that it does not support 5-level EPT in /proc/cpuinfo will make it difficult for users to realize that physical-bits needs to be forcibly limited in the command. But advertising max addressable GPA via ioctl and this patch do not conflict. Thanks, Tao
