Re: [PATCH] KVM: x86/mmu: Always guard rmaps with mmu_lock on PREEMPT_RT=y kernels

[David Woodhouse]

On Wed, 2026-09-02 at 14:17 -0700, Sean Christopherson wrote:
> 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@xxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/8d47b43e1829ac92703723e6a1a4afc7a2eaacb5.camel@xxxxxxxxxxxxx
> Fixes: 4834eaded91e ("KVM: x86/mmu: Add infrastructure to allow walking rmaps outside of mmu_lock")
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Tested-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

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