[GIT PULL 0/1] kvm/s390: Fixes for 6.3

[GIT PULL 0/1] kvm/s390: Fixes for 6.3

[Janosch Frank]

Dear Paolo,

currently we only have one fix patch to offer which repairs the
external loop detection code for PV guests.

Please pull,
Janosch

The following changes since commit 197b6b60ae7bc51dd0814953c562833143b292aa:

  Linux 6.3-rc4 (2023-03-26 14:40:20 -0700)

are available in the Git repository at:

  https://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git tags/kvm-s390-master-6.3-1

for you to fetch changes up to 21f27df854008b86349a203bf97fef79bb11f53e:

  KVM: s390: pv: fix external interruption loop not always detected (2023-03-28 07:16:37 +0000)

Nico Boehr (1):
      KVM: s390: pv: fix external interruption loop not always detected

 arch/s390/kvm/intercept.c | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)
-- 
2.39.2

[GIT PULL 1/1] KVM: s390: pv: fix external interruption loop not always detected

[Janosch Frank]

From: Nico Boehr <nrb@linux.ibm.com>

To determine whether the guest has caused an external interruption loop
upon code 20 (external interrupt) intercepts, the ext_new_psw needs to
be inspected to see whether external interrupts are enabled.

Under non-PV, ext_new_psw can simply be taken from guest lowcore. Under
PV, KVM can only access the encrypted guest lowcore and hence the
ext_new_psw must not be taken from guest lowcore.

handle_external_interrupt() incorrectly did that and hence was not able
to reliably tell whether an external interruption loop is happening or
not. False negatives cause spurious failures of my kvm-unit-test
for extint loops[1] under PV.

Since code 20 is only caused under PV if and only if the guest's
ext_new_psw is enabled for external interrupts, false positive detection
of a external interruption loop can not happen.

Fix this issue by instead looking at the guest PSW in the state
description. Since the PSW swap for external interrupt is done by the
ultravisor before the intercept is caused, this reliably tells whether
the guest is enabled for external interrupts in the ext_new_psw.

Also update the comments to explain better what is happening.

[1] https://lore.kernel.org/kvm/20220812062151.1980937-4-nrb@linux.ibm.com/

Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Fixes: 201ae986ead7 ("KVM: s390: protvirt: Implement interrupt injection")
Link: https://lore.kernel.org/r/20230213085520.100756-2-nrb@linux.ibm.com
Message-Id: <20230213085520.100756-2-nrb@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
---
 arch/s390/kvm/intercept.c | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
index 0ee02dae14b2..2cda8d9d7c6e 100644
--- a/arch/s390/kvm/intercept.c
+++ b/arch/s390/kvm/intercept.c
@@ -271,10 +271,18 @@ static int handle_prog(struct kvm_vcpu *vcpu)
  * handle_external_interrupt - used for external interruption interceptions
  * @vcpu: virtual cpu
  *
- * This interception only occurs if the CPUSTAT_EXT_INT bit was set, or if
- * the new PSW does not have external interrupts disabled. In the first case,
- * we've got to deliver the interrupt manually, and in the second case, we
- * drop to userspace to handle the situation there.
+ * This interception occurs if:
+ * - the CPUSTAT_EXT_INT bit was already set when the external interrupt
+ *   occurred. In this case, the interrupt needs to be injected manually to
+ *   preserve interrupt priority.
+ * - the external new PSW has external interrupts enabled, which will cause an
+ *   interruption loop. We drop to userspace in this case.
+ *
+ * The latter case can be detected by inspecting the external mask bit in the
+ * external new psw.
+ *
+ * Under PV, only the latter case can occur, since interrupt priorities are
+ * handled in the ultravisor.
  */
 static int handle_external_interrupt(struct kvm_vcpu *vcpu)
 {
@@ -285,10 +293,18 @@ static int handle_external_interrupt(struct kvm_vcpu *vcpu)
 
 	vcpu->stat.exit_external_interrupt++;
 
-	rc = read_guest_lc(vcpu, __LC_EXT_NEW_PSW, &newpsw, sizeof(psw_t));
-	if (rc)
-		return rc;
-	/* We can not handle clock comparator or timer interrupt with bad PSW */
+	if (kvm_s390_pv_cpu_is_protected(vcpu)) {
+		newpsw = vcpu->arch.sie_block->gpsw;
+	} else {
+		rc = read_guest_lc(vcpu, __LC_EXT_NEW_PSW, &newpsw, sizeof(psw_t));
+		if (rc)
+			return rc;
+	}
+
+	/*
+	 * Clock comparator or timer interrupt with external interrupt enabled
+	 * will cause interrupt loop. Drop to userspace.
+	 */
 	if ((eic == EXT_IRQ_CLK_COMP || eic == EXT_IRQ_CPU_TIMER) &&
 	    (newpsw.mask & PSW_MASK_EXT))
 		return -EOPNOTSUPP;
-- 
2.39.2

Re: [GIT PULL 0/1] kvm/s390: Fixes for 6.3

[Paolo Bonzini]

Pulled, thanks.

Paolo

On Wed, Mar 29, 2023 at 3:52 PM Janosch Frank <frankja@linux.ibm.com> wrote:
>
> Dear Paolo,
>
> currently we only have one fix patch to offer which repairs the
> external loop detection code for PV guests.
>
> Please pull,
> Janosch
>
> The following changes since commit 197b6b60ae7bc51dd0814953c562833143b292aa:
>
>   Linux 6.3-rc4 (2023-03-26 14:40:20 -0700)
>
> are available in the Git repository at:
>
>   https://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git tags/kvm-s390-master-6.3-1
>
> for you to fetch changes up to 21f27df854008b86349a203bf97fef79bb11f53e:
>
>   KVM: s390: pv: fix external interruption loop not always detected (2023-03-28 07:16:37 +0000)
>
> Nico Boehr (1):
>       KVM: s390: pv: fix external interruption loop not always detected
>
>  arch/s390/kvm/intercept.c | 32 ++++++++++++++++++++++++--------
>  1 file changed, 24 insertions(+), 8 deletions(-)
> --
> 2.39.2
>