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

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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@xxxxxxxxxxxxx>
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?

+			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@xxxxxxxxxxxxx>




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