if KVM is enabled and KVM capabilities MMU radix is available,
the partition table entry (patb_entry) for the radix mode is
initialized by default in ppc_spapr_reset().

It's a problem if we want to migrate the guest to a POWER8 host
while the kernel is not started to set the value to the one
expected for a POWER8 CPU.

The "-machine max-cpu-compat=power8" should allow to migrate
a POWER9 KVM host to a POWER8 KVM host, but because patb_entry
is set, the destination QEMU tries to enable radix mode on the
POWER8 host. This fails and cancels the migration:

    Process table config unsupported by the host
    error while loading state for instance 0x0 of device 'spapr'
    load of migration failed: Invalid argument

This patch doesn't set the PATB entry if the user provides
a CPU compatibility mode that doesn't support radix mode.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 hw/ppc/spapr.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 1ac7eb0f8c..66d0ed5256 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -1440,7 +1440,10 @@ static void ppc_spapr_reset(void)
     /* Check for unknown sysbus devices */
     foreach_dynamic_sysbus_device(find_unknown_sysbus_device, NULL);
 
-    if (kvm_enabled() && kvmppc_has_cap_mmu_radix()) {
+    first_ppc_cpu = POWERPC_CPU(first_cpu);
+    if (kvm_enabled() && kvmppc_has_cap_mmu_radix() &&
+        ppc_check_compat(first_ppc_cpu, CPU_POWERPC_LOGICAL_3_00, 0,
+                         spapr->max_compat_pvr)) {
         /* If using KVM with radix mode available, VCPUs can be started
          * without a HPT because KVM will start them in radix mode.
          * Set the GR bit in PATB so that we know there is no HPT. */
@@ -1499,7 +1502,6 @@ static void ppc_spapr_reset(void)
     g_free(fdt);
 
     /* Set up the entry state */
-    first_ppc_cpu = POWERPC_CPU(first_cpu);
     first_ppc_cpu->env.gpr[3] = fdt_addr;
     first_ppc_cpu->env.gpr[5] = 0;
     first_cpu->halted = 0;
-- 
2.14.3

On Thu, Dec 14, 2017 at 07:09:48PM +0100, Laurent Vivier wrote:
> if KVM is enabled and KVM capabilities MMU radix is available,
> the partition table entry (patb_entry) for the radix mode is
> initialized by default in ppc_spapr_reset().
> 
> It's a problem if we want to migrate the guest to a POWER8 host
> while the kernel is not started to set the value to the one
> expected for a POWER8 CPU.
> 
> The "-machine max-cpu-compat=power8" should allow to migrate
> a POWER9 KVM host to a POWER8 KVM host, but because patb_entry
> is set, the destination QEMU tries to enable radix mode on the
> POWER8 host. This fails and cancels the migration:
> 
>     Process table config unsupported by the host
>     error while loading state for instance 0x0 of device 'spapr'
>     load of migration failed: Invalid argument
> 
> This patch doesn't set the PATB entry if the user provides
> a CPU compatibility mode that doesn't support radix mode.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Applied to ppc-for-2.12.

> ---
>  hw/ppc/spapr.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 1ac7eb0f8c..66d0ed5256 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -1440,7 +1440,10 @@ static void ppc_spapr_reset(void)
>      /* Check for unknown sysbus devices */
>      foreach_dynamic_sysbus_device(find_unknown_sysbus_device, NULL);
>  
> -    if (kvm_enabled() && kvmppc_has_cap_mmu_radix()) {
> +    first_ppc_cpu = POWERPC_CPU(first_cpu);
> +    if (kvm_enabled() && kvmppc_has_cap_mmu_radix() &&
> +        ppc_check_compat(first_ppc_cpu, CPU_POWERPC_LOGICAL_3_00, 0,
> +                         spapr->max_compat_pvr)) {
>          /* If using KVM with radix mode available, VCPUs can be started
>           * without a HPT because KVM will start them in radix mode.
>           * Set the GR bit in PATB so that we know there is no HPT. */
> @@ -1499,7 +1502,6 @@ static void ppc_spapr_reset(void)
>      g_free(fdt);
>  
>      /* Set up the entry state */
> -    first_ppc_cpu = POWERPC_CPU(first_cpu);
>      first_ppc_cpu->env.gpr[3] = fdt_addr;
>      first_ppc_cpu->env.gpr[5] = 0;
>      first_cpu->halted = 0;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson