[PATCH v2 0/3] KVM: allow listener to stop all vcpus before

[PATCH v2 0/3] KVM: allow listener to stop all vcpus before

[Emanuele Giuseppe Esposito]

QEMU needs to perform memslots operations like merging and splitting,
and each operation requires more than a single ioctl.
Therefore if a vcpu is concurrently reading the same memslots,
it could end up reading something that was temporarly deleted.
For example, merging two memslots into one would imply:
DELETE(m1)
DELETE(m2)
CREATE(m1+m2)

And a vcpu could attempt to read m2 right after it is deleted, but
before the new one is created.

This approach is 100% QEMU-based. No KVM API modification is involved,
but implies that QEMU must make sure no new ioctl is running and all
vcpus are stopped.

The logic and code are basically taken from David Hildenbrand
proposal given a while ago while reviewing a previous attempt where
I suggested to solve the above problem directly in KVM by extending
its API.

This is the original code:
https://github.com/davidhildenbrand/qemu/commit/86b1bf546a8d00908e33f7362b0b61e2be8dbb7a

I just split the patch in three smaller patches, and used a
QemuLockCnt instead of counter + mutex.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1979276

Emanuele
---
v2:
* use QemuEvent instead of spinning in ioctl_inhibit_begin
* move the blocker API in a separate file and header, so that other accel can
  use it if they want.

David Hildenbrand (1):
  kvm: Atomic memslot updates

Emanuele Giuseppe Esposito (2):
  accel: introduce accelerator blocker API
  KVM: keep track of running ioctls

 accel/accel-blocker.c          | 139 +++++++++++++++++++++++++++++++++
 accel/kvm/kvm-all.c            | 107 ++++++++++++++++++++++---
 accel/meson.build              |   2 +-
 hw/core/cpu-common.c           |   2 +
 include/hw/core/cpu.h          |   3 +
 include/sysemu/accel-blocker.h |  45 +++++++++++
 include/sysemu/kvm_int.h       |   8 ++
 7 files changed, 294 insertions(+), 12 deletions(-)
 create mode 100644 accel/accel-blocker.c
 create mode 100644 include/sysemu/accel-blocker.h

-- 
2.31.1

[PATCH v2 1/3] accel: introduce accelerator blocker API

[Emanuele Giuseppe Esposito]

This API allows the accelerators to prevent vcpus from issuing
new ioctls while execting a critical section marked with the
accel-ioctl_inhibit_begin/end functions.

Note that all functions submitting ioctls must mark where the
ioctl is being called with accel_{cpu_}set_in_ioctl().

This API requires the caller to always hold the BQL.
API documentation is in sysemu/accel-blocker.h

Internally, it uses a QemuLockCnt together with a per-CPU QemuLockCnt
(to minimize cache line bouncing) to keep avoid that new ioctls
run when the critical section starts, and a QemuEvent to wait
that all running ioctls finish.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 accel/accel-blocker.c          | 139 +++++++++++++++++++++++++++++++++
 accel/meson.build              |   2 +-
 include/sysemu/accel-blocker.h |  45 +++++++++++
 3 files changed, 185 insertions(+), 1 deletion(-)
 create mode 100644 accel/accel-blocker.c
 create mode 100644 include/sysemu/accel-blocker.h

diff --git a/accel/accel-blocker.c b/accel/accel-blocker.c
new file mode 100644
index 0000000000..2701a05945
--- /dev/null
+++ b/accel/accel-blocker.c
@@ -0,0 +1,139 @@
+/*
+ * QEMU accel blocker class
+ *
+ * Copyright (c) 2014 Red Hat Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/thread.h"
+#include "qemu/main-loop.h"
+#include "hw/core/cpu.h"
+#include "sysemu/accel-blocker.h"
+
+static QemuLockCnt accel_in_ioctl_lock;
+static QemuEvent accel_in_ioctl_event;
+
+void accel_blocker_init(void)
+{
+    qemu_lockcnt_init(&accel_in_ioctl_lock);
+    qemu_event_init(&accel_in_ioctl_event, false);
+}
+
+void accel_set_in_ioctl(bool in_ioctl)
+{
+    if (likely(qemu_mutex_iothread_locked())) {
+        return;
+    }
+    if (in_ioctl) {
+        /* block if lock is taken in kvm_ioctl_inhibit_begin() */
+        qemu_lockcnt_inc(&accel_in_ioctl_lock);
+    } else {
+        qemu_lockcnt_dec(&accel_in_ioctl_lock);
+        /* change event to SET. If event was BUSY, wake up all waiters */
+        qemu_event_set(&accel_in_ioctl_event);
+    }
+}
+
+void accel_cpu_set_in_ioctl(CPUState *cpu, bool in_ioctl)
+{
+    if (unlikely(qemu_mutex_iothread_locked())) {
+        return;
+    }
+    if (in_ioctl) {
+        /* block if lock is taken in kvm_ioctl_inhibit_begin() */
+        qemu_lockcnt_inc(&cpu->in_ioctl_lock);
+    } else {
+        qemu_lockcnt_dec(&cpu->in_ioctl_lock);
+        /* change event to SET. If event was BUSY, wake up all waiters */
+        qemu_event_set(&accel_in_ioctl_event);
+    }
+}
+
+static int accel_in_ioctls(void)
+{
+    CPUState *cpu;
+    int ret = qemu_lockcnt_count(&accel_in_ioctl_lock);
+
+    CPU_FOREACH(cpu) {
+        ret += qemu_lockcnt_count(&cpu->in_ioctl_lock);
+    }
+
+    return  ret;
+}
+
+void accel_ioctl_inhibit_begin(void)
+{
+    CPUState *cpu;
+
+    /*
+     * We allow to inhibit only when holding the BQL, so we can identify
+     * when an inhibitor wants to issue an ioctl easily.
+     */
+    g_assert(qemu_mutex_iothread_locked());
+
+    /* Block further invocations of the ioctls outside the BQL.  */
+    CPU_FOREACH(cpu) {
+        qemu_lockcnt_lock(&cpu->in_ioctl_lock);
+    }
+    qemu_lockcnt_lock(&accel_in_ioctl_lock);
+
+    /* Keep waiting until there are running ioctls */
+    while (accel_in_ioctls()) {
+        /* Reset event to FREE. */
+        qemu_event_reset(&accel_in_ioctl_event);
+
+        if (accel_in_ioctls()) {
+
+            CPU_FOREACH(cpu) {
+                /* exit the ioctl */
+                qemu_cpu_kick(cpu);
+            }
+
+            /*
+             * If event is still FREE, and there are ioctls still in progress,
+             * wait.
+             *
+             *  If an ioctl finishes before qemu_event_wait(), it will change
+             * the event state to SET. This will prevent qemu_event_wait() from
+             * blocking, but it's not a problem because if other ioctls are
+             * still running (accel_in_ioctls is true) the loop will iterate
+             * once more and reset the event status to FREE so that it can wait
+             * properly.
+             *
+             * If an ioctls finishes while qemu_event_wait() is blocking, then
+             * it will be waken up, but also here the while loop makes sure
+             * to re-enter the wait if there are other running ioctls.
+             */
+            qemu_event_wait(&accel_in_ioctl_event);
+        }
+    }
+}
+
+void accel_ioctl_inhibit_end(void)
+{
+    CPUState *cpu;
+
+    qemu_lockcnt_unlock(&accel_in_ioctl_lock);
+    CPU_FOREACH(cpu) {
+        qemu_lockcnt_unlock(&cpu->in_ioctl_lock);
+    }
+}
+
diff --git a/accel/meson.build b/accel/meson.build
index b9a963cf80..a0d49c4f31 100644
--- a/accel/meson.build
+++ b/accel/meson.build
@@ -1,4 +1,4 @@
-specific_ss.add(files('accel-common.c'))
+specific_ss.add(files('accel-common.c', 'accel-blocker.c'))
 softmmu_ss.add(files('accel-softmmu.c'))
 user_ss.add(files('accel-user.c'))
 
diff --git a/include/sysemu/accel-blocker.h b/include/sysemu/accel-blocker.h
new file mode 100644
index 0000000000..135ebea566
--- /dev/null
+++ b/include/sysemu/accel-blocker.h
@@ -0,0 +1,45 @@
+/*
+ * Accelerator blocking API, to prevent new ioctls from starting and wait the
+ * running ones finish.
+ * This mechanism differs from pause/resume_all_vcpus() in that it does not
+ * release the BQL.
+ *
+ *  Copyright (c) 2014 Red Hat Inc.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+#ifndef ACCEL_BLOCKER_H
+#define ACCEL_BLOCKER_H
+
+#include "qemu/osdep.h"
+#include "qemu/accel.h"
+#include "sysemu/cpus.h"
+
+extern void accel_blocker_init(void);
+
+/*
+ * accel_set_in_ioctl/accel_cpu_set_in_ioctl:
+ * Mark when ioctl is about to run or just finished.
+ * If @in_ioctl is true, then mark it is beginning. Otherwise marks that it is
+ * ending.
+ *
+ * These functions will block after accel_ioctl_inhibit_begin() is called,
+ * preventing new ioctls to run. They will continue only after
+ * accel_ioctl_inibith_end().
+ */
+extern void accel_set_in_ioctl(bool in_ioctl);
+extern void accel_cpu_set_in_ioctl(CPUState *cpu, bool in_ioctl);
+
+/*
+ * accel_ioctl_inhibit_begin/end: start/end critical section
+ * Between these two calls, no ioctl marked with accel_set_in_ioctl() and
+ * accel_cpu_set_in_ioctl() is allowed to run.
+ *
+ * This allows the caller to access shared data or perform operations without
+ * worrying of concurrent vcpus accesses.
+ */
+extern void accel_ioctl_inhibit_begin(void);
+extern void accel_ioctl_inhibit_end(void);
+
+#endif /* ACCEL_BLOCKER_H */
-- 
2.31.1

Re: [PATCH v2 1/3] accel: introduce accelerator blocker API

[Paolo Bonzini]

On 11/10/22 17:48, Emanuele Giuseppe Esposito wrote:
> +/*
> + * QEMU accel blocker class

"Lock to inhibit accelerator ioctls"

> + *
> + * Copyright (c) 2014 Red Hat Inc.

2022, you can also add an Author line.

> +static int accel_in_ioctls(void)

Return bool (and return early if ret becomes true).

> +void accel_ioctl_inhibit_begin(void)
> +{
> +    CPUState *cpu;
> +
> +    /*
> +     * We allow to inhibit only when holding the BQL, so we can identify
> +     * when an inhibitor wants to issue an ioctl easily.
> +     */
> +    g_assert(qemu_mutex_iothread_locked());
> +
> +    /* Block further invocations of the ioctls outside the BQL.  */
> +    CPU_FOREACH(cpu) {
> +        qemu_lockcnt_lock(&cpu->in_ioctl_lock);
> +    }
> +    qemu_lockcnt_lock(&accel_in_ioctl_lock);
> +
> +    /* Keep waiting until there are running ioctls */
> +    while (accel_in_ioctls()) {
> +        /* Reset event to FREE. */
> +        qemu_event_reset(&accel_in_ioctl_event);
> +
> +        if (accel_in_ioctls()) {
> +
> +            CPU_FOREACH(cpu) {
> +                /* exit the ioctl */
> +                qemu_cpu_kick(cpu);

Only kick if the lockcnt count is > 0? (this is not racy; if it is == 0, 
it cannot ever become > 0 again while the lock is taken)

> diff --git a/include/sysemu/accel-blocker.h b/include/sysemu/accel-blocker.h
> new file mode 100644
> index 0000000000..135ebea566
> --- /dev/null
> +++ b/include/sysemu/accel-blocker.h
> @@ -0,0 +1,45 @@
> +/*
> + * Accelerator blocking API, to prevent new ioctls from starting and wait the
> + * running ones finish.
> + * This mechanism differs from pause/resume_all_vcpus() in that it does not
> + * release the BQL.
> + *
> + *  Copyright (c) 2014 Red Hat Inc.

2022, you can also add an Author line here too.

> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +#ifndef ACCEL_BLOCKER_H
> +#define ACCEL_BLOCKER_H
> +
> +#include "qemu/osdep.h"
> +#include "qemu/accel.h"

qemu/accel.h not needed?

> +#include "sysemu/cpus.h"
> +
> +extern void accel_blocker_init(void);
> +
> +/*
> + * accel_set_in_ioctl/accel_cpu_set_in_ioctl:
> + * Mark when ioctl is about to run or just finished.
> + * If @in_ioctl is true, then mark it is beginning. Otherwise marks that it is
> + * ending.
> + *
> + * These functions will block after accel_ioctl_inhibit_begin() is called,
> + * preventing new ioctls to run. They will continue only after
> + * accel_ioctl_inibith_end().
> + */
> +extern void accel_set_in_ioctl(bool in_ioctl);
> +extern void accel_cpu_set_in_ioctl(CPUState *cpu, bool in_ioctl);

Why not just

extern void accel_ioctl_begin(void);
extern void accel_ioctl_end(void);
extern void accel_cpu_ioctl_begin(CPUState *cpu);
extern void accel_cpu_ioctl_end(CPUState *cpu);

?

Otherwise it's very nice.

Paolo

Re: [PATCH v2 1/3] accel: introduce accelerator blocker API

[Emanuele Giuseppe Esposito]

Am 11/11/2022 um 11:48 schrieb Paolo Bonzini:
> On 11/10/22 17:48, Emanuele Giuseppe Esposito wrote:
>> +/*
>> + * QEMU accel blocker class
> 
> "Lock to inhibit accelerator ioctls"
> 
>> + *
>> + * Copyright (c) 2014 Red Hat Inc.
> 
> 2022, you can also add an Author line.
> 
>> +static int accel_in_ioctls(void)
> 
> Return bool (and return early if ret becomes true).
> 
>> +void accel_ioctl_inhibit_begin(void)
>> +{
>> +    CPUState *cpu;
>> +
>> +    /*
>> +     * We allow to inhibit only when holding the BQL, so we can identify
>> +     * when an inhibitor wants to issue an ioctl easily.
>> +     */
>> +    g_assert(qemu_mutex_iothread_locked());
>> +
>> +    /* Block further invocations of the ioctls outside the BQL.  */
>> +    CPU_FOREACH(cpu) {
>> +        qemu_lockcnt_lock(&cpu->in_ioctl_lock);
>> +    }
>> +    qemu_lockcnt_lock(&accel_in_ioctl_lock);
>> +
>> +    /* Keep waiting until there are running ioctls */
>> +    while (accel_in_ioctls()) {
>> +        /* Reset event to FREE. */
>> +        qemu_event_reset(&accel_in_ioctl_event);
>> +
>> +        if (accel_in_ioctls()) {
>> +
>> +            CPU_FOREACH(cpu) {
>> +                /* exit the ioctl */
>> +                qemu_cpu_kick(cpu);
> 
> Only kick if the lockcnt count is > 0? (this is not racy; if it is == 0,
> it cannot ever become > 0 again while the lock is taken)

Better:

accel_has_to_wait(void)
{
    CPUState *cpu;
    bool needs_to_wait = false;

    CPU_FOREACH(cpu) {
        if (qemu_lockcnt_count(&cpu->in_ioctl_lock)) {
            qemu_cpu_kick(cpu);
            needs_to_wait = true;
        }
    }

    return needs_to_wait || qemu_lockcnt_count(&accel_in_ioctl_lock);
}

And then the loop becomes:

while (true) {
        qemu_event_reset(&accel_in_ioctl_event);

        if (accel_has_to_wait()) {
            qemu_event_wait(&accel_in_ioctl_event);
        } else {
            /* No ioctl is running */
            return;
        }
}

> 
>> diff --git a/include/sysemu/accel-blocker.h
>> b/include/sysemu/accel-blocker.h
>> new file mode 100644
>> index 0000000000..135ebea566
>> --- /dev/null
>> +++ b/include/sysemu/accel-blocker.h
>> @@ -0,0 +1,45 @@
>> +/*
>> + * Accelerator blocking API, to prevent new ioctls from starting and
>> wait the
>> + * running ones finish.
>> + * This mechanism differs from pause/resume_all_vcpus() in that it
>> does not
>> + * release the BQL.
>> + *
>> + *  Copyright (c) 2014 Red Hat Inc.
> 
> 2022, you can also add an Author line here too.
> 
>> + * This work is licensed under the terms of the GNU GPL, version 2 or
>> later.
>> + * See the COPYING file in the top-level directory.
>> + */
>> +#ifndef ACCEL_BLOCKER_H
>> +#define ACCEL_BLOCKER_H
>> +
>> +#include "qemu/osdep.h"
>> +#include "qemu/accel.h"
> 
> qemu/accel.h not needed?
> 
>> +#include "sysemu/cpus.h"
>> +
>> +extern void accel_blocker_init(void);
>> +
>> +/*
>> + * accel_set_in_ioctl/accel_cpu_set_in_ioctl:
>> + * Mark when ioctl is about to run or just finished.
>> + * If @in_ioctl is true, then mark it is beginning. Otherwise marks
>> that it is
>> + * ending.
>> + *
>> + * These functions will block after accel_ioctl_inhibit_begin() is
>> called,
>> + * preventing new ioctls to run. They will continue only after
>> + * accel_ioctl_inibith_end().
>> + */
>> +extern void accel_set_in_ioctl(bool in_ioctl);
>> +extern void accel_cpu_set_in_ioctl(CPUState *cpu, bool in_ioctl);
> 
> Why not just
> 
> extern void accel_ioctl_begin(void);
> extern void accel_ioctl_end(void);
> extern void accel_cpu_ioctl_begin(CPUState *cpu);
> extern void accel_cpu_ioctl_end(CPUState *cpu);
> 
> ?

Ok, makes sense.

Thank you,
Emanuele

> 
> Otherwise it's very nice.
> 
> Paolo
> 

[PATCH v2 2/3] KVM: keep track of running ioctls

[Emanuele Giuseppe Esposito]

Using the new accel-blocker API, mark where ioctls are being called
in KVM. Next, we will implement the critical section that will take
care of performing memslots modifications atomically, therefore
preventing any new ioctl from running and allowing the running ones
to finish.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 accel/kvm/kvm-all.c   | 7 +++++++
 hw/core/cpu-common.c  | 2 ++
 include/hw/core/cpu.h | 3 +++
 3 files changed, 12 insertions(+)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index f99b0becd8..dfc6fe76db 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -2310,6 +2310,7 @@ static int kvm_init(MachineState *ms)
     assert(TARGET_PAGE_SIZE <= qemu_real_host_page_size());
 
     s->sigmask_len = 8;
+    accel_blocker_init();
 
 #ifdef KVM_CAP_SET_GUEST_DEBUG
     QTAILQ_INIT(&s->kvm_sw_breakpoints);
@@ -3014,7 +3015,9 @@ int kvm_vm_ioctl(KVMState *s, int type, ...)
     va_end(ap);
 
     trace_kvm_vm_ioctl(type, arg);
+    accel_set_in_ioctl(true);
     ret = ioctl(s->vmfd, type, arg);
+    accel_set_in_ioctl(false);
     if (ret == -1) {
         ret = -errno;
     }
@@ -3032,7 +3035,9 @@ int kvm_vcpu_ioctl(CPUState *cpu, int type, ...)
     va_end(ap);
 
     trace_kvm_vcpu_ioctl(cpu->cpu_index, type, arg);
+    accel_cpu_set_in_ioctl(cpu, true);
     ret = ioctl(cpu->kvm_fd, type, arg);
+    accel_cpu_set_in_ioctl(cpu, false);
     if (ret == -1) {
         ret = -errno;
     }
@@ -3050,7 +3055,9 @@ int kvm_device_ioctl(int fd, int type, ...)
     va_end(ap);
 
     trace_kvm_device_ioctl(fd, type, arg);
+    accel_set_in_ioctl(true);
     ret = ioctl(fd, type, arg);
+    accel_set_in_ioctl(false);
     if (ret == -1) {
         ret = -errno;
     }
diff --git a/hw/core/cpu-common.c b/hw/core/cpu-common.c
index f9fdd46b9d..8d6a4b1b65 100644
--- a/hw/core/cpu-common.c
+++ b/hw/core/cpu-common.c
@@ -237,6 +237,7 @@ static void cpu_common_initfn(Object *obj)
     cpu->nr_threads = 1;
 
     qemu_mutex_init(&cpu->work_mutex);
+    qemu_lockcnt_init(&cpu->in_ioctl_lock);
     QSIMPLEQ_INIT(&cpu->work_list);
     QTAILQ_INIT(&cpu->breakpoints);
     QTAILQ_INIT(&cpu->watchpoints);
@@ -248,6 +249,7 @@ static void cpu_common_finalize(Object *obj)
 {
     CPUState *cpu = CPU(obj);
 
+    qemu_lockcnt_destroy(&cpu->in_ioctl_lock);
     qemu_mutex_destroy(&cpu->work_mutex);
 }
 
diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index f9b58773f7..15053663bc 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -397,6 +397,9 @@ struct CPUState {
     uint32_t kvm_fetch_index;
     uint64_t dirty_pages;
 
+    /* Use by accel-block: CPU is executing an ioctl() */
+    QemuLockCnt in_ioctl_lock;
+
     /* Used for events with 'vcpu' and *without* the 'disabled' properties */
     DECLARE_BITMAP(trace_dstate_delayed, CPU_TRACE_DSTATE_MAX_EVENTS);
     DECLARE_BITMAP(trace_dstate, CPU_TRACE_DSTATE_MAX_EVENTS);
-- 
2.31.1

Re: [PATCH v2 2/3] KVM: keep track of running ioctls

[Paolo Bonzini]

On 11/10/22 17:48, Emanuele Giuseppe Esposito wrote:
> diff --git a/hw/core/cpu-common.c b/hw/core/cpu-common.c
> index f9fdd46b9d..8d6a4b1b65 100644
> --- a/hw/core/cpu-common.c
> +++ b/hw/core/cpu-common.c
> @@ -237,6 +237,7 @@ static void cpu_common_initfn(Object *obj)
>       cpu->nr_threads = 1;
>   
>       qemu_mutex_init(&cpu->work_mutex);
> +    qemu_lockcnt_init(&cpu->in_ioctl_lock);
>       QSIMPLEQ_INIT(&cpu->work_list);
>       QTAILQ_INIT(&cpu->breakpoints);
>       QTAILQ_INIT(&cpu->watchpoints);
> @@ -248,6 +249,7 @@ static void cpu_common_finalize(Object *obj)
>   {
>       CPUState *cpu = CPU(obj);
>   
> +    qemu_lockcnt_destroy(&cpu->in_ioctl_lock);
>       qemu_mutex_destroy(&cpu->work_mutex);
>   }
> diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
> index f9b58773f7..15053663bc 100644
> --- a/include/hw/core/cpu.h
> +++ b/include/hw/core/cpu.h
> @@ -397,6 +397,9 @@ struct CPUState {
>       uint32_t kvm_fetch_index;
>       uint64_t dirty_pages;
>   
> +    /* Use by accel-block: CPU is executing an ioctl() */
> +    QemuLockCnt in_ioctl_lock;
> +
>       /* Used for events with 'vcpu' and *without* the 'disabled' properties */
>       DECLARE_BITMAP(trace_dstate_delayed, CPU_TRACE_DSTATE_MAX_EVENTS);
>       DECLARE_BITMAP(trace_dstate, CPU_TRACE_DSTATE_MAX_EVENTS);

These go in patch 1.

Paolo

[PATCH v2 3/3] kvm: Atomic memslot updates

[Emanuele Giuseppe Esposito]

From: David Hildenbrand <david@redhat.com>

If we update an existing memslot (e.g., resize, split), we temporarily
remove the memslot to re-add it immediately afterwards. These updates
are not atomic, especially not for KVM VCPU threads, such that we can
get spurious faults.

Let's inhibit most KVM ioctls while performing relevant updates, such
that we can perform the update just as if it would happen atomically
without additional kernel support.

We capture the add/del changes and apply them in the notifier commit
stage instead. There, we can check for overlaps and perform the ioctl
inhibiting only if really required (-> overlap).

To keep things simple we don't perform additional checks that wouldn't
actually result in an overlap -- such as !RAM memory regions in some
cases (see kvm_set_phys_mem()).

To minimize cache-line bouncing, use a separate indicator
(in_ioctl_lock) per CPU.  Also, make sure to hold the kvm_slots_lock
while performing both actions (removing+re-adding).

We have to wait until all IOCTLs were exited and block new ones from
getting executed.

This approach cannot result in a deadlock as long as the inhibitor does
not hold any locks that might hinder an IOCTL from getting finished and
exited - something fairly unusual. The inhibitor will always hold the BQL.

AFAIKs, one possible candidate would be userfaultfd. If a page cannot be
placed (e.g., during postcopy), because we're waiting for a lock, or if the
userfaultfd thread cannot process a fault, because it is waiting for a
lock, there could be a deadlock. However, the BQL is not applicable here,
because any other guest memory access while holding the BQL would already
result in a deadlock.

Nothing else in the kernel should block forever and wait for userspace
intervention.

Note: pause_all_vcpus()/resume_all_vcpus() or
start_exclusive()/end_exclusive() cannot be used, as they either drop
the BQL or require to be called without the BQL - something inhibitors
cannot handle. We need a low-level locking mechanism that is
deadlock-free even when not releasing the BQL.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Tested-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 accel/kvm/kvm-all.c      | 100 ++++++++++++++++++++++++++++++++++-----
 include/sysemu/kvm_int.h |   8 ++++
 2 files changed, 97 insertions(+), 11 deletions(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index dfc6fe76db..ec478dbe69 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -31,6 +31,7 @@
 #include "sysemu/kvm_int.h"
 #include "sysemu/runstate.h"
 #include "sysemu/cpus.h"
+#include "sysemu/accel-blocker.h"
 #include "qemu/bswap.h"
 #include "exec/memory.h"
 #include "exec/ram_addr.h"
@@ -46,6 +47,7 @@
 #include "sysemu/hw_accel.h"
 #include "kvm-cpus.h"
 #include "sysemu/dirtylimit.h"
+#include "qemu/range.h"
 
 #include "hw/boards.h"
 #include "monitor/stats.h"
@@ -1292,6 +1294,7 @@ void kvm_set_max_memslot_size(hwaddr max_slot_size)
     kvm_max_slot_size = max_slot_size;
 }
 
+/* Called with KVMMemoryListener.slots_lock held */
 static void kvm_set_phys_mem(KVMMemoryListener *kml,
                              MemoryRegionSection *section, bool add)
 {
@@ -1326,14 +1329,12 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
     ram = memory_region_get_ram_ptr(mr) + mr_offset;
     ram_start_offset = memory_region_get_ram_addr(mr) + mr_offset;
 
-    kvm_slots_lock();
-
     if (!add) {
         do {
             slot_size = MIN(kvm_max_slot_size, size);
             mem = kvm_lookup_matching_slot(kml, start_addr, slot_size);
             if (!mem) {
-                goto out;
+                return;
             }
             if (mem->flags & KVM_MEM_LOG_DIRTY_PAGES) {
                 /*
@@ -1371,7 +1372,7 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
             start_addr += slot_size;
             size -= slot_size;
         } while (size);
-        goto out;
+        return;
     }
 
     /* register the new slot */
@@ -1396,9 +1397,6 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
         ram += slot_size;
         size -= slot_size;
     } while (size);
-
-out:
-    kvm_slots_unlock();
 }
 
 static void *kvm_dirty_ring_reaper_thread(void *data)
@@ -1455,18 +1453,94 @@ static void kvm_region_add(MemoryListener *listener,
                            MemoryRegionSection *section)
 {
     KVMMemoryListener *kml = container_of(listener, KVMMemoryListener, listener);
+    KVMMemoryUpdate *update;
+
+    update = g_new0(KVMMemoryUpdate, 1);
+    update->section = memory_region_section_new_copy(section);
 
-    memory_region_ref(section->mr);
-    kvm_set_phys_mem(kml, section, true);
+    QSIMPLEQ_INSERT_TAIL(&kml->transaction_add, update, next);
 }
 
 static void kvm_region_del(MemoryListener *listener,
                            MemoryRegionSection *section)
 {
     KVMMemoryListener *kml = container_of(listener, KVMMemoryListener, listener);
+    KVMMemoryUpdate *update;
 
-    kvm_set_phys_mem(kml, section, false);
-    memory_region_unref(section->mr);
+    update = g_new0(KVMMemoryUpdate, 1);
+    update->section = memory_region_section_new_copy(section);
+
+    QSIMPLEQ_INSERT_TAIL(&kml->transaction_del, update, next);
+}
+
+static void kvm_region_commit(MemoryListener *listener)
+{
+    KVMMemoryListener *kml = container_of(listener, KVMMemoryListener,
+                                          listener);
+    KVMMemoryUpdate *u1, *u2;
+    bool need_inhibit = false;
+
+    if (QSIMPLEQ_EMPTY(&kml->transaction_add) &&
+        QSIMPLEQ_EMPTY(&kml->transaction_del)) {
+        return;
+    }
+
+    /*
+     * We have to be careful when regions to add overlap with ranges to remove.
+     * We have to simulate atomic KVM memslot updates by making sure no ioctl()
+     * is currently active.
+     *
+     * The lists are order by addresses, so it's easy to find overlaps.
+     */
+    u1 = QSIMPLEQ_FIRST(&kml->transaction_del);
+    u2 = QSIMPLEQ_FIRST(&kml->transaction_add);
+    while (u1 && u2) {
+        Range r1, r2;
+
+        range_init_nofail(&r1, u1->section->offset_within_address_space,
+                          int128_get64(u1->section->size));
+        range_init_nofail(&r2, u2->section->offset_within_address_space,
+                          int128_get64(u2->section->size));
+
+        if (range_overlaps_range(&r1, &r2)) {
+            need_inhibit = true;
+            break;
+        }
+        if (range_lob(&r1) < range_lob(&r2)) {
+            u1 = QSIMPLEQ_NEXT(u1, next);
+        } else {
+            u2 = QSIMPLEQ_NEXT(u2, next);
+        }
+    }
+
+
+    kvm_slots_lock();
+    if (need_inhibit) {
+        accel_ioctl_inhibit_begin();
+    }
+
+    /* Remove all memslots before adding the new ones. */
+    QSIMPLEQ_FOREACH_SAFE(u1, &kml->transaction_del, next, u2) {
+        kvm_set_phys_mem(kml, u1->section, false);
+        memory_region_unref(u1->section->mr);
+
+        QSIMPLEQ_REMOVE(&kml->transaction_del, u1, KVMMemoryUpdate, next);
+        memory_region_section_free_copy(u1->section);
+        g_free(u1);
+    }
+    QSIMPLEQ_FOREACH_SAFE(u1, &kml->transaction_add, next, u2) {
+        memory_region_ref(u1->section->mr);
+        kvm_set_phys_mem(kml, u1->section, true);
+
+        QSIMPLEQ_REMOVE(&kml->transaction_add, u1, KVMMemoryUpdate, next);
+        memory_region_section_free_copy(u1->section);
+        g_free(u1);
+    }
+
+    if (need_inhibit) {
+        accel_ioctl_inhibit_end();
+    }
+    kvm_slots_unlock();
 }
 
 static void kvm_log_sync(MemoryListener *listener,
@@ -1610,8 +1684,12 @@ void kvm_memory_listener_register(KVMState *s, KVMMemoryListener *kml,
         kml->slots[i].slot = i;
     }
 
+    QSIMPLEQ_INIT(&kml->transaction_add);
+    QSIMPLEQ_INIT(&kml->transaction_del);
+
     kml->listener.region_add = kvm_region_add;
     kml->listener.region_del = kvm_region_del;
+    kml->listener.commit = kvm_region_commit;
     kml->listener.log_start = kvm_log_start;
     kml->listener.log_stop = kvm_log_stop;
     kml->listener.priority = 10;
diff --git a/include/sysemu/kvm_int.h b/include/sysemu/kvm_int.h
index 3b4adcdc10..5ea2d7924b 100644
--- a/include/sysemu/kvm_int.h
+++ b/include/sysemu/kvm_int.h
@@ -12,6 +12,7 @@
 #include "exec/memory.h"
 #include "qapi/qapi-types-common.h"
 #include "qemu/accel.h"
+#include "qemu/queue.h"
 #include "sysemu/kvm.h"
 
 typedef struct KVMSlot
@@ -31,10 +32,17 @@ typedef struct KVMSlot
     ram_addr_t ram_start_offset;
 } KVMSlot;
 
+typedef struct KVMMemoryUpdate {
+    QSIMPLEQ_ENTRY(KVMMemoryUpdate) next;
+    MemoryRegionSection *section;
+} KVMMemoryUpdate;
+
 typedef struct KVMMemoryListener {
     MemoryListener listener;
     KVMSlot *slots;
     int as_id;
+    QSIMPLEQ_HEAD(, KVMMemoryUpdate) transaction_add;
+    QSIMPLEQ_HEAD(, KVMMemoryUpdate) transaction_del;
 } KVMMemoryListener;
 
 #define KVM_MSI_HASHTAB_SIZE    256
-- 
2.31.1

Re: [PATCH v2 3/3] kvm: Atomic memslot updates

[Paolo Bonzini]

On 11/10/22 17:48, Emanuele Giuseppe Esposito wrote:
> 
> +    /* Remove all memslots before adding the new ones. */
> +    QSIMPLEQ_FOREACH_SAFE(u1, &kml->transaction_del, next, u2) {
> +        kvm_set_phys_mem(kml, u1->section, false);
> +        memory_region_unref(u1->section->mr);
> +
> +        QSIMPLEQ_REMOVE(&kml->transaction_del, u1, KVMMemoryUpdate, next);
> +        memory_region_section_free_copy(u1->section);
> +        g_free(u1);
> +    }
> +    QSIMPLEQ_FOREACH_SAFE(u1, &kml->transaction_add, next, u2) {
> +        memory_region_ref(u1->section->mr);
> +        kvm_set_phys_mem(kml, u1->section, true);
> +
> +        QSIMPLEQ_REMOVE(&kml->transaction_add, u1, KVMMemoryUpdate, next);
> +        memory_region_section_free_copy(u1->section);
> +        g_free(u1);
> +    }

I'm not a huge fan of new_copy/free_copy, and I don't think it's needed 
here.  The FlatView is certainly alive between begin and commit (see 
address_space_set_flatview()), and the MemoryRegion is kept alive by the 
FlatView.  In other words, unlike other uses of the functions, here the 
lifetime is bound by kvm_region_commit, and that means you can just copy 
by value.  You can just copy it into KVMMemoryUpdate, i.e.

     typedef struct KVMMemoryUpdate {
         QSIMPLEQ_ENTRY(KVMMemoryUpdate) next;
         MemoryRegionSection section;
     } KVMMemoryUpdate;

Also, you can write the loop as

     while (!QSIMPLEQ_EMPTY(&kvm->transaction_add) {
         u = QSIMPLEQ_FIRST(&kvm->transaction_add);
         QSIMPLEQ_REMOVE_HEAD(&kml->transaction_add, next);
         ...
         g_free(u);
     }

This clarifies the invariant that the QSIMPLEQs become empty at the end 
of the loop.  If it were QTAILQ it would be more a matter of personal 
taste, but QSIMPLEQ_REMOVE is technically not constant-time and that 
also tilts in favor of QSIMPLEQ_REMOVE_HEAD.

In fact I think we should remove QSIMPLEQ_REMOVE and QSLIST_REMOVE, 
changing all users that need it to QTAILQ and QLIST respectively... 
added to https://wiki.qemu.org/Contribute/BiteSizedTasks#API_conversion.

Paolo