http://bugzilla.opensuse.org/show_bug.cgi?id=1174374
http://bugzilla.opensuse.org/show_bug.cgi?id=1174374#c10
--- Comment #10 from Liang Yan ---
Did a further research, the default of cap-ccf-assist is off but changed by
below commit in qemu-5.0.
===============
commit 37965dfe4dffa3ac49438337417608e7f346b58a
Author: David Gibson
Date: Thu Jan 30 10:28:56 2020 +1100
spapr: Enable DD2.3 accelerated count cache flush in pseries-5.0 machine
For POWER9 DD2.2 cpus, the best current Spectre v2 indirect branch
mitigation is "count cache disabled", which is configured with:
-machine cap-ibs=fixed-ccd
However, this option isn't available on DD2.3 CPUs with KVM, because they
don't have the count cache disabled.
+ uint8_t kvm_ibs = kvmppc_get_cap_safe_indirect_branch();
+
+ if (kvm_ibs == SPAPR_CAP_FIXED_CCD) {
+ /*
+ * If we don't have CCF assist on the host, the assist
+ * instruction is a harmless no-op. It won't correctly
+ * implement the cache count flush *but* if we have
+ * count-cache-disabled in the host, that flush is
+ * unnnecessary. So, specifically allow this case. This
+ * allows us to have better performance on POWER9 DD2.3,
+ * while still working on POWER9 DD2.2 and POWER8 host
+ * cpus.
+ */
+ return;
+ }
error_setg(errp,
"Requested count cache flush assist capability level not supported by kvm,"
" try appending -machine cap-ccf-assist=off");
}
According to above code, it seems KVM host does not support
count-cache-disabled. Because cap-ccf-assist will be recongnized as a no-op if
it supports.
You may need to update host firmware to support it.
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