Patch "KVM: PPC: Book3S HV: Don't dynamically split core when already split" has been added to the 4.3-stable tree

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

KVM: PPC: Book3S HV: Don't dynamically split core when already split

to the 4.3-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-ppc-book3s-hv-don-t-dynamically-split-core-when-already-split.patch
and it can be found in the queue-4.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Paul Mackerras <[email protected]>
Date: Tue, 3 Nov 2015 16:03:30 +1100
Subject: KVM: PPC: Book3S HV: Don't dynamically split core when already split

From: Paul Mackerras <[email protected]>

commit f74f2e2e26199f695ca3df94f29e9ab7cb707ea4 upstream.

In static micro-threading modes, the dynamic micro-threading code is supposed to be disabled, because subcores can't make independent decisions about what micro-threading mode to put the core in - there is only one micro-threading mode for the whole core. The code that implements dynamic micro-threading checks for this, except that the check was missed in one case. This means that it is possible for a subcore in static 2-way micro-threading mode to try to put the core into 4-way micro-threading mode, which usually leads to stuck CPUs, spinlock lockups, and other stalls in the host.

The problem was in the can_split_piggybacked_subcores() function, which should always return false if the system is in a static micro-threading mode. This fixes the problem by making can_split_piggybacked_subcores() use subcore_config_ok() for its checks, as subcore_config_ok() includes the necessary check for the static micro-threading modes.

Credit to Gautham Shenoy for working out that the reason for the hangs and stalls we were seeing was that we were trying to do dynamic 4-way micro-threading while we were in static 2-way mode.

---
 arch/powerpc/kvm/book3s_hv.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -2019,7 +2019,7 @@ static bool can_split_piggybacked_subcor
 			return false;
 		n_subcores += (cip->subcore_threads[sub] - 1) >> 1;
 	}
-	if (n_subcores > 3 || large_sub < 0)
+	if (large_sub < 0 || !subcore_config_ok(n_subcores + 1, 2))
 		return false;
 
 	/*

Patches currently in stable-queue which might be from [email protected] are

queue-4.3/kvm-ppc-book3s-hv-prohibit-setting-illegal-transaction-state-in-msr.patch queue-4.3/kvm-ppc-book3s-hv-don-t-dynamically-split-core-when-already-split.patch