Re: [PATCH v1 1/1] KVM: s390: pv: fix asynchronous teardown for small VMs

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Fri, 21 Apr 2023 10:07:22 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> Am 20.04.23 um 18:01 schrieb Claudio Imbrenda:
> > On machines without the Destroy Secure Configuration Fast UVC, the
> > topmost level of page tables is set aside and freed asynchronously
> > as last step of the asynchronous teardown.
> > 
> > Each gmap has a host_to_guest radix tree mapping host (userspace)
> > addresses (with 1M granularity) to gmap segment table entries (pmds).
> > 
> > If a guest is smaller than 2GB, the topmost level of page tables is the
> > segment table (i.e. there are only 2 levels). Replacing it means that
> > the pointers in the host_to_guest mapping would become stale and cause
> > all kinds of nasty issues.
> > 
> > This patch fixes the issue by synchronously destroying all guests with
> > only 2 levels of page tables in kvm_s390_pv_set_aside. This will
> > speed up the process and avoid the issue altogether.
> > 
> > Update s390_replace_asce so it refuses to replace segment type ASCEs.
> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > Fixes: fb491d5500a7 ("KVM: s390: pv: asynchronous destroy for reboot")
> > ---
> >   arch/s390/kvm/pv.c  | 35 ++++++++++++++++++++---------------
> >   arch/s390/mm/gmap.c |  7 +++++++
> >   2 files changed, 27 insertions(+), 15 deletions(-)
> > 
> > diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
> > index e032ebbf51b9..ceb8cb628d62 100644
> > --- a/arch/s390/kvm/pv.c
> > +++ b/arch/s390/kvm/pv.c
> > @@ -39,6 +39,7 @@ struct pv_vm_to_be_destroyed {
> >   	u64 handle;
> >   	void *stor_var;
> >   	unsigned long stor_base;
> > +	bool small;
> >   };
> >   
> >   static void kvm_s390_clear_pv_state(struct kvm *kvm)
> > @@ -318,7 +319,11 @@ int kvm_s390_pv_set_aside(struct kvm *kvm, u16 *rc, u16 *rrc)
> >   	if (!priv)
> >   		return -ENOMEM;
> >   
> > -	if (is_destroy_fast_available()) {
> > +	if ((kvm->arch.gmap->asce & _ASCE_TYPE_MASK) == _ASCE_TYPE_SEGMENT) {
> > +		/* No need to do things asynchronously for VMs under 2GB */
> > +		res = kvm_s390_pv_deinit_vm(kvm, rc, rrc);
> > +		priv->small = true;
> > +	} else if (is_destroy_fast_available()) {
> >   		res = kvm_s390_pv_deinit_vm_fast(kvm, rc, rrc);
> >   	} else {
> >   		priv->stor_var = kvm->arch.pv.stor_var;
> > @@ -335,7 +340,8 @@ int kvm_s390_pv_set_aside(struct kvm *kvm, u16 *rc, u16 *rrc)
> >   		return res;
> >   	}
> >   
> > -	kvm_s390_destroy_lower_2g(kvm);
> > +	if (!priv->small)
> > +		kvm_s390_destroy_lower_2g(kvm);
> >   	kvm_s390_clear_pv_state(kvm);
> >   	kvm->arch.pv.set_aside = priv;
> >   
> > @@ -418,7 +424,10 @@ int kvm_s390_pv_deinit_cleanup_all(struct kvm *kvm, u16 *rc, u16 *rrc)
> >   
> >   	/* If a previous protected VM was set aside, put it in the need_cleanup list */
> >   	if (kvm->arch.pv.set_aside) {
> > -		list_add(kvm->arch.pv.set_aside, &kvm->arch.pv.need_cleanup);
> > +		if (((struct pv_vm_to_be_destroyed *)kvm->arch.pv.set_aside)->small)  
> why do we need a cast here?

it's a void *

but it's not important, after talking with Marc I have found a much
simpler solution (will post soon)

> 
> > +			kfree(kvm->arch.pv.set_aside);
> > +		else
> > +			list_add(kvm->arch.pv.set_aside, &kvm->arch.pv.need_cleanup);
> >   		kvm->arch.pv.set_aside = NULL;
> >   	}
> >     
> 
> With the comment added that Marc asked for
> 
> Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
>