Patch "KVM: s390: pv: fix external interruption loop not always detected" has been added to the 5.10-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    KVM: s390: pv: fix external interruption loop not always detected

to the 5.10-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-s390-pv-fix-external-interruption-loop-not-alway.patch
and it can be found in the queue-5.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit bdacc99e0a8069bbb1038b00d732183b34771205
Author: Nico Boehr <nrb@xxxxxxxxxxxxx>
Date:   Mon Feb 13 09:55:20 2023 +0100

    KVM: s390: pv: fix external interruption loop not always detected
    
    [ Upstream commit 21f27df854008b86349a203bf97fef79bb11f53e ]
    
    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@xxxxxxxxxxxxx/
    
    Signed-off-by: Nico Boehr <nrb@xxxxxxxxxxxxx>
    Reviewed-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
    Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
    Fixes: 201ae986ead7 ("KVM: s390: protvirt: Implement interrupt injection")
    Link: https://lore.kernel.org/r/20230213085520.100756-2-nrb@xxxxxxxxxxxxx
    Message-Id: <20230213085520.100756-2-nrb@xxxxxxxxxxxxx>
    Signed-off-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
index 77909d362b78f..5be68190901f9 100644
--- a/arch/s390/kvm/intercept.c
+++ b/arch/s390/kvm/intercept.c
@@ -270,10 +270,18 @@ static int handle_prog(struct kvm_vcpu *vcpu)
 /**
  * handle_external_interrupt - used for external interruption interceptions
  *
- * 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)
 {
@@ -284,10 +292,18 @@ 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;