Re: [PATCH 1/4] KVM: x86/mmu: Reload MMU on *every* page pre-fault attempt/iteration

On Thu, 2026-08-06 at 14:40 -0700, Sean Christopherson wrote:
> Reload the MMU (which is a nop if the MMU doesn't need to be reloaded) on
> every attempt to pre-fault a guest page, i.e. when the page fault path
> signals that the caller should retry.  If the synchronize_srcu_expedited()
> in kvm_invalidate_memslot() completes before kvm_vcpu_pre_fault_memory()
> grabs SRCU, but kvm_mmu_reload() in the pre-fault path completes before
> kvm_invalidate_memslot() triggers x86's "fast zap all", then the pre-fault
> task will reach kvm_tdp_page_prefault() with an invalid root.
> 
> Attempting to fault-in memory with an invalid root ultimately puts
> kvm_tdp_page_prefault() into an infinite (breakable) retry loop, which
> manifests most obviously as a hang in the pre_fault_memory_test selftest,
> but also eventually causes RCU (SRCU?) to complain.
> 
>   INFO: rcu_tasks detected stalls on tasks:
>   000000000cda47bd: .. nvcsw: 6/6 holdout: 1 idle_cpu: -1/25
>   task:pre_fault_memor state:R  running task     stack:12696
>   pid:95588 tgid:95588 ppid:95584  task_flags:0x400000 flags:0x00080801
>   Call Trace:
>    <TASK>
>    lock_release+0x4e/0x320
>    __get_user_pages+0x546/0xcd0
>    up_read+0x1b/0x30
>    get_user_pages_unlocked+0xee/0x350
>    hva_to_pfn+0xd3/0x3d0 [kvm]
>    lock_release+0x4e/0x320
>    xa_load+0x5c/0x170
>    xa_load+0x14c/0x170
>    __kvm_faultin_pfn+0xd9/0x130 [kvm]
>    lock_acquire+0x65/0x2b0
>    lock_release+0x4e/0x320
>    kvm_mmu_faultin_pfn+0x1e1/0x690 [kvm]
>    gup_fast_fallback+0x63e/0xdf0
>    kvm_tdp_page_fault+0xeb/0x140 [kvm]
>    kvm_mmu_do_page_fault+0x12e/0x200 [kvm]
>    kvm_arch_vcpu_pre_fault_memory+0x16e/0x200 [kvm]
>    kvm_vcpu_pre_fault_memory+0xc1/0x1f0 [kvm]
>    kvm_vcpu_pre_fault_memory+0x116/0x1f0 [kvm]
>    kvm_vcpu_ioctl+0x3a4/0x6b0 [kvm]
>    clockevents_program_event+0x5d/0x170
>    __se_sys_ioctl+0x6d/0xb0
>    entry_SYSCALL_64_after_hwframe+0x4b/0x53
>    do_syscall_64+0x10a/0x480
>    __irq_exit_rcu+0x8e/0x140
>    entry_SYSCALL_64_after_hwframe+0x4b/0x53
>    </TASK>
> 
> Fixes: 6e01b7601dfe ("KVM: x86: Implement kvm_arch_vcpu_pre_fault_memory()")
> Signed-off-by: Sean Christopherson <[email protected]>
> 

Reviewed-by: Kai Huang <[email protected]>