[PATCH 1/5] KVM: arm64: Walk userspace page tables to compute the THP mapping size

[Will Deacon]

Hey Sean,

On Tue, Jul 20, 2021 at 08:33:46PM +0000, Sean Christopherson wrote:
> On Tue, Jul 20, 2021, Alexandru Elisei wrote:
> > I just can't figure out why having the mmap lock is not needed to walk the
> > userspace page tables. Any hints? Or am I not seeing where it's taken?
> 
> Disclaimer: I'm not super familiar with arm64's page tables, but the relevant KVM
> functionality is common across x86 and arm64.

No need for the disclaimer, there are so many moving parts here that I don't
think it's possible to be familiar with them all! Thanks for taking the time
to write it up so clearly.

> KVM arm64 (and x86) unconditionally registers a mmu_notifier for the mm_struct
> associated with the VM, and disallows calling ioctls from a different process,
> i.e. walking the page tables during KVM_RUN is guaranteed to use the mm for which
> KVM registered the mmu_notifier.  As part of registration, the mmu_notifier
> does mmgrab() and doesn't do mmdrop() until it's unregistered.  That ensures the
> mm_struct itself is live.
> 
> For the page tables liveliness, KVM implements mmu_notifier_ops.release, which is
> invoked at the beginning of exit_mmap(), before the page tables are freed.  In
> its implementation, KVM takes mmu_lock and zaps all its shadow page tables, a.k.a.
> the stage2 tables in KVM arm64.  The flow in question, get_user_mapping_size(),
> also runs under mmu_lock, and so effectively blocks exit_mmap() and thus is
> guaranteed to run with live userspace tables.

Unless I missed a case, exit_mmap() only runs when mm_struct::mm_users drops
to zero, right? The vCPU tasks should hold references to that afaict, so I
don't think it should be possible for exit_mmap() to run while there are
vCPUs running with the corresponding page-table.

> Looking at the arm64 code, one thing I'm not clear on is whether arm64 correctly
> handles the case where exit_mmap() wins the race.  The invalidate_range hooks will
> still be called, so userspace page tables aren't a problem, but
> kvm_arch_flush_shadow_all() -> kvm_free_stage2_pgd() nullifies mmu->pgt without
> any additional notifications that I see.  x86 deals with this by ensuring its
> top-level TDP entry (stage2 equivalent) is valid while the page fault handler is
> running.

But the fact that x86 handles this race has me worried. What am I missing?

I agree that, if the race can occur, we don't appear to handle it in the
arm64 backend.

Cheers,

Will