Commit c363a37a authored by Daniel Henrique Barboza's avatar Daniel Henrique Barboza Committed by David Gibson
Browse files

target/ppc: 'PVR != host PVR' in KVM_SET_SREGS workaround 

Commit d5fc133e ("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: default avatarDaniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
[dwg: Fix for the case of using TCG on a PPC host]
Signed-off-by: default avatarDavid Gibson <david@gibson.dropbear.id.au>
parent b96919d7

target/ppc/kvm.c

+38 −0
Original line number Diff line number Diff line
@@ -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 +2793,33 @@ 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 (!kvm_enabled()) {

        return false;

    }



    if (cap_ppc_pvr_compat) {

        return false;

    }



    return !kvmppc_is_pr(cs->kvm_state);

}

target/ppc/kvm_ppc.h

+1 −0
Original line number Diff line number Diff line
@@ -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);

target/ppc/machine.c

+22 −0
Original line number Diff line number Diff line
@@ -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)

{
@@ -249,6 +250,27 @@ 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]);