Re: [PATCH] x86/cpu: Add a VMX flag to enumerate 5-level EPT support to userspace

[Sean Christopherson <seanjc@google.com>]

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...

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.