Re: [Qemu-devel] [Qemu-ppc] [PATCH] target/ppc: 'PVR != host PVR' in KVM_SET_SREGS workaround

[Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>]

On 08/09/2017 09:52 PM, David Gibson wrote:
> 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 <danielhb@linux.vnet.ibm.com>
> Ugly, but necessary.  Applied to ppc-for-2.10.
Definitely not my finest hour either.

Hopefully when we start looking at P8 -> P9 migrations we can clean this up
or make it less ugly.


Daniel

>
>> ---
>>   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]);