[Qemu-devel] [PATCH 2/3] spapr: compute default value of "hpt-max-page-size" later

[Greg Kurz]

It is currently not possible to run a pseries-2.12 or older machine
with HV KVM. QEMU prints the following and exits right away.

qemu-system-ppc64: KVM doesn't support for base page shift 34

The "hpt-max-page-size" capability was recently added to spapr to hide
host configuration details from HPT mode guests. Its default value for
newer machine types is 64k.

For backwards compatibility, pseries-2.12 and older machine types need
a different value. This is handled as usual in a class init function.
The default value is 16G, ie, all page sizes supported by POWER7 and
newer CPUs, but HV KVM requires guest pages to be hpa contiguous as
well as gpa contiguous. The default value is the page size used to
back the guest RAM in this case.

Unfortunately kvmppc_hpt_needs_host_contiguous_pages()->kvm_enabled() is
called way before KVM init and returns false, even if the user requested
KVM. We thus end up selecting 16G, which isn't supported by HV KVM.

We fix this by moving the logic to spapr_machine_init() because this
is the earliest call where we're sure kvm_enabled() can be trusted.
Since the user cannot pass cap-hpt-max-page-size=0, we set the default
to 0 in the pseries-2.12 class init function and use that as a flag
in spapr_machine_init() to do the real work.

Signed-off-by: Greg Kurz <gr...@kaod.org>
---
 hw/ppc/spapr.c |   25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 8cc996d0b822..1eb45cd8c424 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -2527,6 +2527,18 @@ static void spapr_machine_init(MachineState *machine)
     QLIST_INIT(&spapr->phbs);
     QTAILQ_INIT(&spapr->pending_dimm_unplugs);
 
+    /* This is for pseries-2.12 and older machine types */
+    if (!smc->default_caps.caps[SPAPR_CAP_HPT_MAXPAGESIZE]) {
+        uint8_t mps;
+
+        if (kvmppc_hpt_needs_host_contiguous_pages()) {
+            mps = ctz64(qemu_getrampagesize());
+        } else {
+            mps = 34; /* allow everything up to 16GiB, i.e. everything */
+        }
+        smc->default_caps.caps[SPAPR_CAP_HPT_MAXPAGESIZE] = mps;
+    }
+
     /* Determine capabilities to run with */
     spapr_caps_init(spapr);
 
@@ -4103,17 +4115,16 @@ static void 
spapr_machine_2_12_instance_options(MachineState *machine)
 static void spapr_machine_2_12_class_options(MachineClass *mc)
 {
     sPAPRMachineClass *smc = SPAPR_MACHINE_CLASS(mc);
-    uint8_t mps;
 
     spapr_machine_3_0_class_options(mc);
     SET_MACHINE_COMPAT(mc, SPAPR_COMPAT_2_12);
 
-    if (kvmppc_hpt_needs_host_contiguous_pages()) {
-        mps = ctz64(qemu_getrampagesize());
-    } else {
-        mps = 34; /* allow everything up to 16GiB, i.e. everything */
-    }
-    smc->default_caps.caps[SPAPR_CAP_HPT_MAXPAGESIZE] = mps;
+    /* We depend on kvm_enabled() to choose a default value for the
+     * hpt-max-page-size capability. Of course we can't do it here
+     * because this is too early and the HW accelerator isn't initialzed
+     * yet. Postpone this to spapr_machine_init().
+     */
+    smc->default_caps.caps[SPAPR_CAP_HPT_MAXPAGESIZE] = 0;
 }
 
 DEFINE_SPAPR_MACHINE(2_12, "2.12", false);