Re: [PATCH 1/3] KVM: Batch setting of per-page memory attributes to avoid soft lockup

[Sean Christopherson <seanjc@google.com>]

On Thu, Jun 05, 2025, Liam Merwick wrote:
> When booting an SEV-SNP guest with a sufficiently large amount of memory (1TB+),
> the host can experience CPU soft lockups when running an operation in
> kvm_vm_set_mem_attributes() to set memory attributes on the whole
> range of guest memory.
> 
> watchdog: BUG: soft lockup - CPU#8 stuck for 26s! [qemu-kvm:6372]
> CPU: 8 UID: 0 PID: 6372 Comm: qemu-kvm Kdump: loaded Not tainted 6.15.0-rc7.20250520.el9uek.rc1.x86_64 #1 PREEMPT(voluntary)
> Hardware name: Oracle Corporation ORACLE SERVER E4-2c/Asm,MB Tray,2U,E4-2c, BIOS 78016600 11/13/2024
> RIP: 0010:xas_create+0x78/0x1f0
> Code: 00 00 00 41 80 fc 01 0f 84 82 00 00 00 ba 06 00 00 00 bd 06 00 00 00 49 8b 45 08 4d 8d 65 08 41 39 d6 73 20 83 ed 06 48 85 c0 <74> 67 48 89 c2 83 e2 03 48 83 fa 02 75 0c 48 3d 00 10 00 00 0f 87
> RSP: 0018:ffffad890a34b940 EFLAGS: 00000286
> RAX: ffff96f30b261daa RBX: ffffad890a34b9c8 RCX: 0000000000000000
> RDX: 000000000000001e RSI: 0000000000000000 RDI: 0000000000000000
> RBP: 0000000000000018 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: ffffad890a356868
> R13: ffffad890a356860 R14: 0000000000000000 R15: ffffad890a356868
> FS:  00007f5578a2a400(0000) GS:ffff97ed317e1000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f015c70fb18 CR3: 00000001109fd006 CR4: 0000000000f70ef0
> PKRU: 55555554
> Call Trace:
>  <TASK>
>  xas_store+0x58/0x630

Trim the '?' lines when including a backtrace in a changelog, they're pure noise.

>  __xa_store+0xa5/0x130
>  xa_store+0x2c/0x50
>  kvm_vm_set_mem_attributes+0x343/0x710 [kvm]
>  kvm_vm_ioctl+0x796/0xab0 [kvm]
>  __x64_sys_ioctl+0xa3/0xd0
>  do_syscall_64+0x8c/0x7a0
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> RIP: 0033:0x7f5578d031bb
> Code: ff ff ff 85 c0 79 9b 49 c7 c4 ff ff ff ff 5b 5d 4c 89 e0 41 5c c3 66 0f 1f 84 00 00 00 00 00 f3 0f 1e fa b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 2d 4c 0f 00 f7 d8 64 89 01 48
> RSP: 002b:00007ffe0a742b88 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 000000004020aed2 RCX: 00007f5578d031bb
> RDX: 00007ffe0a742c80 RSI: 000000004020aed2 RDI: 000000000000000b
> RBP: 0000010000000000 R08: 0000010000000000 R09: 0000017680000000
> R10: 0000000000000080 R11: 0000000000000246 R12: 00005575e5f95120
> R13: 00007ffe0a742c80 R14: 0000000000000008 R15: 00005575e5f961e0
> 
> Limit the range of memory per operation when setting the attributes to
> avoid holding kvm->slots_lock for too long and causing a cpu soft lockup.

Holding slots_lock is totally fine.  Presumably the issue is that the CPU never
reschedules.

E.g. I would expect this to make the problem go away, though it's probably not a
complete fix (I'm guessing kvm_range_has_memory_attributes() can be made to yell
too).

I'd strongly prefer to avoid arbitrary batching, because that raises a bunch of
questions that are difficult to answer, e.g. what guarantees 512GiB is a "good"
batch size on _all_ systems.

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b24db92e98f3..28230bad43f4 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2513,6 +2513,8 @@ static int kvm_vm_set_mem_attributes(struct kvm *kvm, gfn_t start, gfn_t end,
                r = xa_reserve(&kvm->mem_attr_array, i, GFP_KERNEL_ACCOUNT);
                if (r)
                        goto out_unlock;
+
+               cond_resched();
        }
 
        kvm_handle_gfn_range(kvm, &pre_set_range);
@@ -2521,6 +2523,7 @@ static int kvm_vm_set_mem_attributes(struct kvm *kvm, gfn_t start, gfn_t end,
                r = xa_err(xa_store(&kvm->mem_attr_array, i, entry,
                                    GFP_KERNEL_ACCOUNT));
                KVM_BUG_ON(r, kvm);
+               cond_resched();
        }
 
        kvm_handle_gfn_range(kvm, &post_set_range);