Subject: Re: [RFC Qemu PATCH v2 1/2] spapr: drc: Add support for async hcalls at the drc level

From: Greg Kurz <[email-masked]>
Date: Mon, 21 Dec 2020 15:37:50 +0100

On Mon, 21 Dec 2020 13:08:53 +0100
Greg Kurz <[email-masked]> wrote:

> Hi Shiva,
> 
> On Mon, 30 Nov 2020 09:16:39 -0600
> Shivaprasad G Bhat <[email-masked]> wrote:
> 
> > The patch adds support for async hcalls at the DRC level for the
> > spapr devices. To be used by spapr-scm devices in the patch/es to follow.
> > 
> > Signed-off-by: Shivaprasad G Bhat <[email-masked]>
> > ---
> 
> The overall idea looks good but I think you should consider using
> a thread pool to implement it. See below.
> 

Some more comments.

> >  hw/ppc/spapr_drc.c         |  149 ++++++++++++++++++++++++++++++++++++++++++++
> >  include/hw/ppc/spapr_drc.h |   25 +++++++
> >  2 files changed, 174 insertions(+)
> > 
> > diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c
> > index 77718cde1f..4ecd04f686 100644
> > --- a/hw/ppc/spapr_drc.c
> > +++ b/hw/ppc/spapr_drc.c
> > @@ -15,6 +15,7 @@
> >  #include "qapi/qmp/qnull.h"
> >  #include "cpu.h"
> >  #include "qemu/cutils.h"
> > +#include "qemu/guest-random.h"
> >  #include "hw/ppc/spapr_drc.h"
> >  #include "qom/object.h"
> >  #include "migration/vmstate.h"
> > @@ -421,6 +422,148 @@ void spapr_drc_detach(SpaprDrc *drc)
> >      spapr_drc_release(drc);
> >  }
> >  
> > +
> > +/*
> > + * @drc : device DRC targetting which the async hcalls to be made.
> > + *
> > + * All subsequent requests to run/query the status should use the
> > + * unique token returned here.
> > + */
> > +uint64_t spapr_drc_get_new_async_hcall_token(SpaprDrc *drc)
> > +{
> > +    Error *err = NULL;
> > +    uint64_t token;
> > +    SpaprDrcDeviceAsyncHCallState *tmp, *next, *state;
> > +
> > +    state = g_malloc0(sizeof(*state));
> > +    state->pending = true;
> > +
> > +    qemu_mutex_lock(&drc->async_hcall_states_lock);
> > +retry:
> > +    if (qemu_guest_getrandom(&token, sizeof(token), &err) < 0) {
> > +        error_report_err(err);
> > +        g_free(state);
> > +        qemu_mutex_unlock(&drc->async_hcall_states_lock);
> > +        return 0;
> > +    }
> > +
> > +    if (!token) /* Token should be non-zero */
> > +        goto retry;
> > +
> > +    if (!QLIST_EMPTY(&drc->async_hcall_states)) {
> > +        QLIST_FOREACH_SAFE(tmp, &drc->async_hcall_states, node, next) {
> > +            if (tmp->continue_token == token) {
> > +                /* If the token already in use, get a new one */
> > +                goto retry;
> > +            }
> > +        }
> > +    }
> > +
> > +    state->continue_token = token;
> > +    QLIST_INSERT_HEAD(&drc->async_hcall_states, state, node);
> > +
> > +    qemu_mutex_unlock(&drc->async_hcall_states_lock);
> > +
> > +    return state->continue_token;
> > +}
> > +
> > +static void *spapr_drc_async_hcall_runner(void *opaque)
> > +{
> > +    int response = -1;

It feels wrong since the return value of func() is supposed to be
opaque to this function. And anyway it isn't needed since response
is always set a few lines below.

> > +    SpaprDrcDeviceAsyncHCallState *state = opaque;
> > +
> > +    /*
> > +     * state is freed only after this thread finishes(after pthread_join()),
> > +     * don't worry about it becoming NULL.
> > +     */
> > +
> > +    response = state->func(state->data);
> > +
> > +    state->hcall_ret = response;
> > +    state->pending = 0;

s/0/false/

> > +
> > +    return NULL;
> > +}
> > +
> > +/*
> > + * @drc  : device DRC targetting which the async hcalls to be made.
> > + * token : The continue token to be used for tracking as recived from
> > + *         spapr_drc_get_new_async_hcall_token
> > + * @func() : the worker function which needs to be executed asynchronously
> > + * @data : data to be passed to the asynchronous function. Worker is supposed
> > + *         to free/cleanup the data that is passed here
> 
> It'd be cleaner to pass a completion callback and have free/cleanup handled there.
> 
> > + */
> > +void spapr_drc_run_async_hcall(SpaprDrc *drc, uint64_t token,
> > +                               SpaprDrcAsyncHcallWorkerFunc *func, void *data)
> > +{
> > +    SpaprDrcDeviceAsyncHCallState *state;
> > +
> > +    qemu_mutex_lock(&drc->async_hcall_states_lock);
> > +    QLIST_FOREACH(state, &drc->async_hcall_states, node) {
> > +        if (state->continue_token == token) {
> > +            state->func = func;
> > +            state->data = data;
> > +            qemu_thread_create(&state->thread, "sPAPR Async HCALL",
> > +                               spapr_drc_async_hcall_runner, state,
> > +                               QEMU_THREAD_JOINABLE);
> 
> qemu_thread_create() exits on failure, it shouldn't be called on
> a guest triggerable path, eg. a buggy guest could call it up to
> the point that pthread_create() returns EAGAIN.
> 
> Please use a thread pool (see thread_pool_submit_aio()). This takes care
> of all the thread housekeeping for you in a safe way, and it provides a
> completion callback API. The implementation could then be just about
> having two lists: one for pending requests (fed here) and one for
> completed requests (fed by the completion callback).
> 
> > +            break;
> > +        }
> > +    }
> > +    qemu_mutex_unlock(&drc->async_hcall_states_lock);
> > +}
> > +
> > +/*
> > + * spapr_drc_finish_async_hcalls
> > + *      Waits for all pending async requests to complete
> > + *      thier execution and free the states
> > + */
> > +static void spapr_drc_finish_async_hcalls(SpaprDrc *drc)
> > +{
> > +    SpaprDrcDeviceAsyncHCallState *state, *next;
> > +
> > +    if (QLIST_EMPTY(&drc->async_hcall_states)) {
> > +        return;
> > +    }
> > +

This is called during machine reset and there won't be contention
in the large majority of cases. If the list is empty QLIST_FOREACH_SAFE()
won't iterate. So I don't think special casing the empty list brings much.

> > +    qemu_mutex_lock(&drc->async_hcall_states_lock);
> > +    QLIST_FOREACH_SAFE(state, &drc->async_hcall_states, node, next) {
> > +        qemu_thread_join(&state->thread);
> 
> With a thread-pool, you'd just need to aio_poll() until the pending list
> is empty and then clear the completed list.
> 
> > +        QLIST_REMOVE(state, node);
> > +        g_free(state);
> > +    }
> > +    qemu_mutex_unlock(&drc->async_hcall_states_lock);
> > +}
> > +
> > +/*
> > + * spapr_drc_get_async_hcall_status
> > + *      Fetches the status of the hcall worker and returns H_BUSY
> > + *      if the worker is still running.
> > + */
> > +int spapr_drc_get_async_hcall_status(SpaprDrc *drc, uint64_t token)
> > +{
> > +    int ret = H_PARAMETER;
> > +    SpaprDrcDeviceAsyncHCallState *state, *node;
> > +
> > +    qemu_mutex_lock(&drc->async_hcall_states_lock);
> > +    QLIST_FOREACH_SAFE(state, &drc->async_hcall_states, node, node) {
> > +        if (state->continue_token == token) {
> > +            if (state->pending) {
> > +                ret = H_BUSY;
> > +                break;
> > +            } else {
> > +                ret = state->hcall_ret;
> > +                qemu_thread_join(&state->thread);
> 
> Like for qemu_thread_create(), the guest shouldn't be responsible for
> thread housekeeping. Getting the hcall status should just be about
> finding the token in the pending or completed lists.
> 
> > +                QLIST_REMOVE(state, node);
> > +                g_free(state);
> > +                break;
> > +            }
> > +        }
> > +    }
> > +    qemu_mutex_unlock(&drc->async_hcall_states_lock);
> > +
> > +    return ret;
> > +}
> > +
> >  void spapr_drc_reset(SpaprDrc *drc)
> >  {
> >      SpaprDrcClass *drck = SPAPR_DR_CONNECTOR_GET_CLASS(drc);
> > @@ -448,6 +591,7 @@ void spapr_drc_reset(SpaprDrc *drc)
> >          drc->ccs_offset = -1;
> >          drc->ccs_depth = -1;
> >      }
> > +    spapr_drc_finish_async_hcalls(drc);
> >  }
> >  
> >  static bool spapr_drc_unplug_requested_needed(void *opaque)
> > @@ -558,6 +702,7 @@ SpaprDrc *spapr_dr_connector_new(Object *owner, const char *type,
> >      drc->owner = owner;
> >      prop_name = g_strdup_printf("dr-connector[%"PRIu32"]",
> >                                  spapr_drc_index(drc));
> > +
> 
> Unrelated change.
> 
> >      object_property_add_child(owner, prop_name, OBJECT(drc));
> >      object_unref(OBJECT(drc));
> >      qdev_realize(DEVICE(drc), NULL, NULL);
> > @@ -577,6 +722,10 @@ static void spapr_dr_connector_instance_init(Object *obj)
> >      object_property_add(obj, "fdt", "struct", prop_get_fdt,
> >                          NULL, NULL, NULL);
> >      drc->state = drck->empty_state;
> > +
> > +    qemu_mutex_init(&drc->async_hcall_states_lock);
> > +    QLIST_INIT(&drc->async_hcall_states);
> > +
> 
> Empty line not needed.
> 
> >  }
> >  
> >  static void spapr_dr_connector_class_init(ObjectClass *k, void *data)
> > diff --git a/include/hw/ppc/spapr_drc.h b/include/hw/ppc/spapr_drc.h
> > index 165b281496..77f6e4386c 100644
> > --- a/include/hw/ppc/spapr_drc.h
> > +++ b/include/hw/ppc/spapr_drc.h
> > @@ -18,6 +18,7 @@
> >  #include "sysemu/runstate.h"
> >  #include "hw/qdev-core.h"
> >  #include "qapi/error.h"
> > +#include "block/thread-pool.h"
> >  
> >  #define TYPE_SPAPR_DR_CONNECTOR "spapr-dr-connector"
> >  #define SPAPR_DR_CONNECTOR_GET_CLASS(obj) \
> > @@ -168,6 +169,21 @@ typedef enum {
> >      SPAPR_DRC_STATE_PHYSICAL_CONFIGURED = 8,
> >  } SpaprDrcState;
> >  
> > +typedef struct SpaprDrc SpaprDrc;
> > +
> > +typedef int SpaprDrcAsyncHcallWorkerFunc(void *opaque);
> > +typedef struct SpaprDrcDeviceAsyncHCallState {
> > +    uint64_t continue_token;
> > +    bool pending;
> > +
> > +    int hcall_ret;
> > +    SpaprDrcAsyncHcallWorkerFunc *func;
> > +    void *data;
> > +
> > +    QemuThread thread;
> > +
> > +    QLIST_ENTRY(SpaprDrcDeviceAsyncHCallState) node;
> > +} SpaprDrcDeviceAsyncHCallState;
> >  typedef struct SpaprDrc {
> >      /*< private >*/
> >      DeviceState parent;
> > @@ -182,6 +198,10 @@ typedef struct SpaprDrc {
> >      int ccs_offset;
> >      int ccs_depth;
> >  
> > +    /* async hcall states */
> > +    QemuMutex async_hcall_states_lock;
> > +    QLIST_HEAD(, SpaprDrcDeviceAsyncHCallState) async_hcall_states;
> > +
> >      /* device pointer, via link property */
> >      DeviceState *dev;
> >      bool unplug_requested;
> > @@ -241,6 +261,11 @@ void spapr_drc_detach(SpaprDrc *drc);
> >  /* Returns true if a hot plug/unplug request is pending */
> >  bool spapr_drc_transient(SpaprDrc *drc);
> >  
> > +uint64_t spapr_drc_get_new_async_hcall_token(SpaprDrc *drc);
> > +void spapr_drc_run_async_hcall(SpaprDrc *drc, uint64_t token,
> > +                               SpaprDrcAsyncHcallWorkerFunc, void *data);
> > +int spapr_drc_get_async_hcall_status(SpaprDrc *drc, uint64_t token);
> > +
> >  static inline bool spapr_drc_unplug_requested(SpaprDrc *drc)
> >  {
> >      return drc->unplug_requested;
> > 
> > 
> > 
> 
>