KVM: x86/mmu: Always guard rmaps with mmu_lock on PREEMPT_RT=y kernels

For all intents and purposes, revert KVM's ability to walk rmaps outside of
mmu_lock when running on a realtime (PREEMPT_RT=y) kernel.  I.e. don't use
a non-sleepable bit-spinlock to protect rmap entries, as realtime kernels
are highly unlikely to benefit from increased aging throughput and reduced
jitter for memory-overcommitted nested VMs, whereas using a non-sleepable
lock is currently buggy and goes against the spirit of realtime kernels.

Because KVM's rmap locks are hand-crafted bit-spinlocks, preemption must be
disabled before acquiring the lock, otherwise a preempted lock holder will
result in all other walkers of the locked rmap to spin and wait, with no
tracked owner for PI to boost.  For non-RT kernels, acquiring mmu_lock
suffices, as mmu_lock is a non-sleepable rwlock.  But on RT, where mmu_lock
becomes sleepable, preemption is left enabled for rmap writers:

  WARNING: arch/x86/kvm/mmu/mmu.c:920 at __kvm_rmap_lock+0x1a7/0x1e0 [kvm], CPU#16: vmx_apic_update/3708
  CPU: 16 UID: 0 PID: 3708 Comm: vmx_apic_update Not tainted 7.2.0-rc7 #52 PREEMPT_{RT,LAZY}
  RIP: 0010:__kvm_rmap_lock+0x1a7/0x1e0 [kvm]
  Call Trace:
   pte_list_add+0x67/0x4d0 [kvm]
   __link_shadow_page+0x249/0x480 [kvm]
   ept_fetch+0x4d5/0x1220 [kvm]
   ept_page_fault+0x60b/0x850 [kvm]
   kvm_mmu_do_page_fault+0x252/0x690 [kvm]

Alternatively, KVM could manually disable preemption when grabbing an rmap
lock, but as above, that isn't what RT kernels generally want, and it's
actually more complex to implement (cleanly).

To not completely lose the scaling advantage of per-rmap locks, take
mmu_lock for read in the aging path, i.e. allow multiple concurrent aging
tasks, as the aging code needs to use atomic SPTE accesses no matter what,
i.e. no extra code/work is required to guard against concurrent aging of
SPTEs.

Reported-by: David Woodhouse <dwmw2@infradead.org>
Closes: https://lore.kernel.org/all/8d47b43e1829ac92703723e6a1a4afc7a2eaacb5.camel@infradead.org
Fixes: 4834eaded91e ("KVM: x86/mmu: Add infrastructure to allow walking rmaps outside of mmu_lock")
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Tested-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: James Houghton <jthoughton@google.com>
Link: https://patch.msgid.link/20260902211759.2700289-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions