Re: [patch] KVM: SVM: Periodically schedule when unregistering regions on destroy

[Paolo Bonzini <pbonzini@redhat.com>]

On 25/08/20 21:56, David Rientjes wrote:
> There may be many encrypted regions that need to be unregistered when a
> SEV VM is destroyed.  This can lead to soft lockups.  For example, on a
> host running 4.15:
> 
> watchdog: BUG: soft lockup - CPU#206 stuck for 11s! [t_virtual_machi:194348]
> CPU: 206 PID: 194348 Comm: t_virtual_machi
> RIP: 0010:free_unref_page_list+0x105/0x170
> ...
> Call Trace:
>  [<0>] release_pages+0x159/0x3d0
>  [<0>] sev_unpin_memory+0x2c/0x50 [kvm_amd]
>  [<0>] __unregister_enc_region_locked+0x2f/0x70 [kvm_amd]
>  [<0>] svm_vm_destroy+0xa9/0x200 [kvm_amd]
>  [<0>] kvm_arch_destroy_vm+0x47/0x200
>  [<0>] kvm_put_kvm+0x1a8/0x2f0
>  [<0>] kvm_vm_release+0x25/0x30
>  [<0>] do_exit+0x335/0xc10
>  [<0>] do_group_exit+0x3f/0xa0
>  [<0>] get_signal+0x1bc/0x670
>  [<0>] do_signal+0x31/0x130
> 
> Although the CLFLUSH is no longer issued on every encrypted region to be
> unregistered, there are no other changes that can prevent soft lockups for
> very large SEV VMs in the latest kernel.
> 
> Periodically schedule if necessary.  This still holds kvm->lock across the
> resched, but since this only happens when the VM is destroyed this is
> assumed to be acceptable.
> 
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  arch/x86/kvm/svm/sev.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1106,6 +1106,7 @@ void sev_vm_destroy(struct kvm *kvm)
>  		list_for_each_safe(pos, q, head) {
>  			__unregister_enc_region_locked(kvm,
>  				list_entry(pos, struct enc_region, list));
> +			cond_resched();
>  		}
>  	}
>  
> 

Queued, thanks.  Sorry for the delay.

I am currently on leave so I am going through the patches and queuing
them, but I will only push kvm/next and kvm/queue next week.  kvm/master
patches will be sent to Linus for the next -rc though.

Paolo