[Xen-changelog] [qemu-xen master] ppc: Change Power9 compat table to support at most 8 threads/core

[patchbot@xxxxxxx]

commit a1f33a5b93f75abfdbaf4023cf9b7935285a705f
Author:     Jose Ricardo Ziviani <joserz@xxxxxxxxxxxxxxxxxx>
AuthorDate: Sun Jan 14 17:23:48 2018 -0200
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Mon Feb 5 18:55:26 2018 -0600

    ppc: Change Power9 compat table to support at most 8 threads/core
    
    Increases the max smt mode to 8 for Power9. That's because KVM supports
    smt emulation in this platform so QEMU should allow users to use it as
    well.
    
    Today if we try to pass -smp ...,threads=8, QEMU will silently truncate
    it to smt4 mode and may cause a crash if we try to perform a cpu
    hotplug.
    
    Signed-off-by: Jose Ricardo Ziviani <joserz@xxxxxxxxxxxxxxxxxx>
    [dwg: Added an explanatory comment]
    Signed-off-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
    
    (cherry picked from commit 03ee51d3548f5f553a3089f466483c1c6d5c666b)
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 target/ppc/compat.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/target/ppc/compat.c b/target/ppc/compat.c
index ad8f93c..276b5b5 100644
--- a/target/ppc/compat.c
+++ b/target/ppc/compat.c
@@ -73,7 +73,14 @@ static const CompatInfo compat_table[] = {
         .pvr = CPU_POWERPC_LOGICAL_3_00,
         .pcr = PCR_COMPAT_3_00,
         .pcr_level = PCR_COMPAT_3_00,
-        .max_threads = 4,
+        /*
+         * POWER9 hardware only supports 4 threads / core, but this
+         * limit is for guests.  We need to support 8 vthreads/vcore
+         * on POWER9 for POWER8 compatibility guests, and it's very
+         * confusing if half of the threads disappear from the guest
+         * if it announces it's POWER9 aware at CAS time.
+         */
+        .max_threads = 8,
     },
 };
 
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#master

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