On Wed, Aug 09, 2017 at 05:43:46PM -0300, Daniel Henrique Barboza wrote:
> Commit d5fc133eed ("ppc: Rework CPU compatibility testing
> across migration") changed the way cpu_post_load behaves with
> the PVR setting, causing an unexpected bug in KVM-HV migrations
> between hosts that are compatible (POWER8 and POWER8E, for example).
> Even with pvr_match() returning true, the guest freezes right after
> cpu_post_load. The reason is that the guest kernel can't handle a
> different PVR value other that the running host in KVM_SET_SREGS.
>
> In [1] it was discussed the possibility of a new KVM capability
> that would indicate that the guest kernel can handle a different
> PVR in KVM_SET_SREGS. Even if such feature is implemented, there is
> still the problem with older kernels that will not have this capability
> and will fail to migrate.
>
> This patch implements a workaround for that scenario. If running
> with KVM, check if the guest kernel does not have the capability
> (named here as 'cap_ppc_pvr_compat'). If it doesn't, calls
> kvmppc_is_pr() to see if the guest is running in KVM-HV. If all this
> happens, set env->spr[SPR_PVR] to the same value as the current
> host PVR. This ensures that we allow migrations with 'close enough'
> PVRs to still work in KVM-HV but also makes the code ready for
> this new KVM capability when it is done.
>
> A new function called 'kvmppc_pvr_workaround_required' was created
> to encapsulate the conditions said above and to avoid calling too
> many kvm.c internals inside cpu_post_load.
>
> [1] https://lists.gnu.org/archive/html/qemu-ppc/2017-06/msg00503.html
>
> Signed-off-by: Daniel Henrique Barboza <danie...@linux.vnet.ibm.com>
Ugly, but necessary. Applied to ppc-for-2.10.
> ---
> target/ppc/kvm.c | 34 ++++++++++++++++++++++++++++++++++
> target/ppc/kvm_ppc.h | 1 +
> target/ppc/machine.c | 22 ++++++++++++++++++++++
> 3 files changed, 57 insertions(+)
>
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 8571379..fb3ee43 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -90,6 +90,7 @@ static int cap_htm; /* Hardware transactional
> memory support */
> static int cap_mmu_radix;
> static int cap_mmu_hash_v3;
> static int cap_resize_hpt;
> +static int cap_ppc_pvr_compat;
>
> static uint32_t debug_inst_opcode;
>
> @@ -147,6 +148,13 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
> cap_mmu_radix = kvm_vm_check_extension(s, KVM_CAP_PPC_MMU_RADIX);
> cap_mmu_hash_v3 = kvm_vm_check_extension(s, KVM_CAP_PPC_MMU_HASH_V3);
> cap_resize_hpt = kvm_vm_check_extension(s, KVM_CAP_SPAPR_RESIZE_HPT);
> + /*
> + * Note: setting it to false because there is not such capability
> + * in KVM at this moment.
> + *
> + * TODO: call kvm_vm_check_extension() with the right capability
> + * after the kernel starts implementing it.*/
> + cap_ppc_pvr_compat = false;
>
> if (!cap_interrupt_level) {
> fprintf(stderr, "KVM: Couldn't find level irq capability. Expect the
> "
> @@ -2785,3 +2793,29 @@ void kvmppc_update_sdr1(target_ulong sdr1)
> run_on_cpu(cs, kvmppc_pivot_hpt_cpu, RUN_ON_CPU_TARGET_PTR(sdr1));
> }
> }
> +
> +/*
> + * This is a helper function to detect a post migration scenario
> + * in which a guest, running as KVM-HV, freezes in cpu_post_load because
> + * the guest kernel can't handle a PVR value other than the actual host
> + * PVR in KVM_SET_SREGS, even if pvr_match() returns true.
> + *
> + * If we don't have cap_ppc_pvr_compat and we're not running in PR
> + * (so, we're HV), return true. The workaround itself is done in
> + * cpu_post_load.
> + *
> + * The order here is important: we'll only check for KVM PR as a
> + * fallback if the guest kernel can't handle the situation itself.
> + * We need to avoid as much as possible querying the running KVM type
> + * in QEMU level.
> + */
> +bool kvmppc_pvr_workaround_required(PowerPCCPU *cpu)
> +{
> + CPUState *cs = CPU(cpu);
> +
> + if (cap_ppc_pvr_compat) {
> + return false;
> + }
> +
> + return !kvmppc_is_pr(cs->kvm_state);
> +}
> diff --git a/target/ppc/kvm_ppc.h b/target/ppc/kvm_ppc.h
> index 6bc6fb3..381afe6 100644
> --- a/target/ppc/kvm_ppc.h
> +++ b/target/ppc/kvm_ppc.h
> @@ -67,6 +67,7 @@ void kvmppc_check_papr_resize_hpt(Error **errp);
> int kvmppc_resize_hpt_prepare(PowerPCCPU *cpu, target_ulong flags, int
> shift);
> int kvmppc_resize_hpt_commit(PowerPCCPU *cpu, target_ulong flags, int shift);
> void kvmppc_update_sdr1(target_ulong sdr1);
> +bool kvmppc_pvr_workaround_required(PowerPCCPU *cpu);
>
> bool kvmppc_is_mem_backend_page_size_ok(const char *obj_path);
>
> diff --git a/target/ppc/machine.c b/target/ppc/machine.c
> index f578156..e545885 100644
> --- a/target/ppc/machine.c
> +++ b/target/ppc/machine.c
> @@ -9,6 +9,7 @@
> #include "mmu-hash64.h"
> #include "migration/cpu.h"
> #include "qapi/error.h"
> +#include "kvm_ppc.h"
>
> static int cpu_load_old(QEMUFile *f, void *opaque, int version_id)
> {
> @@ -250,6 +251,27 @@ static int cpu_post_load(void *opaque, int version_id)
> }
> }
>
> + /*
> + * If we're running with KVM HV, there is a chance that the guest
> + * is running with KVM HV and its kernel does not have the
> + * capability of dealing with a different PVR other than this
> + * exact host PVR in KVM_SET_SREGS. If that happens, the
> + * guest freezes after migration.
> + *
> + * The function kvmppc_pvr_workaround_required does this verification
> + * by first checking if the kernel has the cap, returning true
> immediately
> + * if that is the case. Otherwise, it checks if we're running in KVM PR.
> + * If the guest kernel does not have the cap and we're not running KVM-PR
> + * (so, it is running KVM-HV), we need to ensure that KVM_SET_SREGS will
> + * receive the PVR it expects as a workaround.
> + *
> + */
> +#if defined(CONFIG_KVM)
> + if (kvmppc_pvr_workaround_required(cpu)) {
> + env->spr[SPR_PVR] = env->spr_cb[SPR_PVR].default_value;
> + }
> +#endif
> +
> env->lr = env->spr[SPR_LR];
> env->ctr = env->spr[SPR_CTR];
> cpu_write_xer(env, env->spr[SPR_XER]);
--
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
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