After system startup, run the update to memory_dispatch
and the tlb_flush on the cpu. This eliminates a race,
wherein a running cpu sees the memory_dispatch change
but has not yet seen the tlb_flush.
Since the update now happens on the cpu, we need not use
qatomic_rcu_read to protect the read of memory_dispatch.
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1826
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1834
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1846
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
softmmu/physmem.c | 40 +++++++++++++++++++++++++++++-----------
1 file changed, 29 insertions(+), 11 deletions(-)
diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index 7597dc1c39..18277ddd67 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -680,8 +680,7 @@ address_space_translate_for_iotlb(CPUState *cpu, int asidx, hwaddr orig_addr,
IOMMUTLBEntry iotlb;
int iommu_idx;
hwaddr addr = orig_addr;
- AddressSpaceDispatch *d =
- qatomic_rcu_read(&cpu->cpu_ases[asidx].memory_dispatch);
+ AddressSpaceDispatch *d = cpu->cpu_ases[asidx].memory_dispatch;
for (;;) {
section = address_space_translate_internal(d, addr, &addr, plen, false);
@@ -2412,7 +2411,7 @@ MemoryRegionSection *iotlb_to_section(CPUState *cpu,
{
int asidx = cpu_asidx_from_attrs(cpu, attrs);
CPUAddressSpace *cpuas = &cpu->cpu_ases[asidx];
- AddressSpaceDispatch *d = qatomic_rcu_read(&cpuas->memory_dispatch);
+ AddressSpaceDispatch *d = cpuas->memory_dispatch;
int section_index = index & ~TARGET_PAGE_MASK;
MemoryRegionSection *ret;
@@ -2487,23 +2486,42 @@ static void tcg_log_global_after_sync(MemoryListener *listener)
}
}
+static void tcg_commit_cpu(CPUState *cpu, run_on_cpu_data data)
+{
+ CPUAddressSpace *cpuas = data.host_ptr;
+
+ cpuas->memory_dispatch = address_space_to_dispatch(cpuas->as);
+ tlb_flush(cpu);
+}
+
static void tcg_commit(MemoryListener *listener)
{
CPUAddressSpace *cpuas;
- AddressSpaceDispatch *d;
+ CPUState *cpu;
assert(tcg_enabled());
/* since each CPU stores ram addresses in its TLB cache, we must
reset the modified entries */
cpuas = container_of(listener, CPUAddressSpace, tcg_as_listener);
- cpu_reloading_memory_map();
- /* The CPU and TLB are protected by the iothread lock.
- * We reload the dispatch pointer now because cpu_reloading_memory_map()
- * may have split the RCU critical section.
+ cpu = cpuas->cpu;
+
+ /*
+ * Defer changes to as->memory_dispatch until the cpu is quiescent.
+ * Otherwise we race between (1) other cpu threads and (2) ongoing
+ * i/o for the current cpu thread, with data cached by mmu_lookup().
+ *
+ * In addition, queueing the work function will kick the cpu back to
+ * the main loop, which will end the RCU critical section and reclaim
+ * the memory data structures.
+ *
+ * That said, the listener is also called during realize, before
+ * all of the tcg machinery for run-on is initialized: thus halt_cond.
*/
- d = address_space_to_dispatch(cpuas->as);
- qatomic_rcu_set(&cpuas->memory_dispatch, d);
- tlb_flush(cpuas->cpu);
+ if (cpu->halt_cond) {
+ async_run_on_cpu(cpu, tcg_commit_cpu, RUN_ON_CPU_HOST_PTR(cpuas));
+ } else {
+ tcg_commit_cpu(cpu, RUN_ON_CPU_HOST_PTR(cpuas));
+ }
}
static void memory_map_init(void)
--
2.34.1On Sat, 26 Aug 2023 16:24:14 -0700
Richard Henderson <richard.henderson@linaro.org> wrote:
> After system startup, run the update to memory_dispatch
> and the tlb_flush on the cpu. This eliminates a race,
> wherein a running cpu sees the memory_dispatch change
> but has not yet seen the tlb_flush.
>
> Since the update now happens on the cpu, we need not use
> qatomic_rcu_read to protect the read of memory_dispatch.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1826
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1834
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1846
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
I'm not pretending I've understood the change though, just that
it makes the crashes I saw go away.
Jonathan
> ---
> softmmu/physmem.c | 40 +++++++++++++++++++++++++++++-----------
> 1 file changed, 29 insertions(+), 11 deletions(-)
>
> diff --git a/softmmu/physmem.c b/softmmu/physmem.c
> index 7597dc1c39..18277ddd67 100644
> --- a/softmmu/physmem.c
> +++ b/softmmu/physmem.c
> @@ -680,8 +680,7 @@ address_space_translate_for_iotlb(CPUState *cpu, int asidx, hwaddr orig_addr,
> IOMMUTLBEntry iotlb;
> int iommu_idx;
> hwaddr addr = orig_addr;
> - AddressSpaceDispatch *d =
> - qatomic_rcu_read(&cpu->cpu_ases[asidx].memory_dispatch);
> + AddressSpaceDispatch *d = cpu->cpu_ases[asidx].memory_dispatch;
>
> for (;;) {
> section = address_space_translate_internal(d, addr, &addr, plen, false);
> @@ -2412,7 +2411,7 @@ MemoryRegionSection *iotlb_to_section(CPUState *cpu,
> {
> int asidx = cpu_asidx_from_attrs(cpu, attrs);
> CPUAddressSpace *cpuas = &cpu->cpu_ases[asidx];
> - AddressSpaceDispatch *d = qatomic_rcu_read(&cpuas->memory_dispatch);
> + AddressSpaceDispatch *d = cpuas->memory_dispatch;
> int section_index = index & ~TARGET_PAGE_MASK;
> MemoryRegionSection *ret;
>
> @@ -2487,23 +2486,42 @@ static void tcg_log_global_after_sync(MemoryListener *listener)
> }
> }
>
> +static void tcg_commit_cpu(CPUState *cpu, run_on_cpu_data data)
> +{
> + CPUAddressSpace *cpuas = data.host_ptr;
> +
> + cpuas->memory_dispatch = address_space_to_dispatch(cpuas->as);
> + tlb_flush(cpu);
> +}
> +
> static void tcg_commit(MemoryListener *listener)
> {
> CPUAddressSpace *cpuas;
> - AddressSpaceDispatch *d;
> + CPUState *cpu;
>
> assert(tcg_enabled());
> /* since each CPU stores ram addresses in its TLB cache, we must
> reset the modified entries */
> cpuas = container_of(listener, CPUAddressSpace, tcg_as_listener);
> - cpu_reloading_memory_map();
> - /* The CPU and TLB are protected by the iothread lock.
> - * We reload the dispatch pointer now because cpu_reloading_memory_map()
> - * may have split the RCU critical section.
> + cpu = cpuas->cpu;
> +
> + /*
> + * Defer changes to as->memory_dispatch until the cpu is quiescent.
> + * Otherwise we race between (1) other cpu threads and (2) ongoing
> + * i/o for the current cpu thread, with data cached by mmu_lookup().
> + *
> + * In addition, queueing the work function will kick the cpu back to
> + * the main loop, which will end the RCU critical section and reclaim
> + * the memory data structures.
> + *
> + * That said, the listener is also called during realize, before
> + * all of the tcg machinery for run-on is initialized: thus halt_cond.
> */
> - d = address_space_to_dispatch(cpuas->as);
> - qatomic_rcu_set(&cpuas->memory_dispatch, d);
> - tlb_flush(cpuas->cpu);
> + if (cpu->halt_cond) {
> + async_run_on_cpu(cpu, tcg_commit_cpu, RUN_ON_CPU_HOST_PTR(cpuas));
> + } else {
> + tcg_commit_cpu(cpu, RUN_ON_CPU_HOST_PTR(cpuas));
> + }
> }
>
> static void memory_map_init(void)
On 8/26/23 16:24, Richard Henderson wrote:
> +static void tcg_commit_cpu(CPUState *cpu, run_on_cpu_data data)
> +{
> + CPUAddressSpace *cpuas = data.host_ptr;
> +
> + cpuas->memory_dispatch = address_space_to_dispatch(cpuas->as);
> + tlb_flush(cpu);
> +}
Question: do I need to take the iothread lock here, while re-generating the address space
dispatch?
r~
Richard Henderson <richard.henderson@linaro.org> writes:
> On 8/26/23 16:24, Richard Henderson wrote:
>> +static void tcg_commit_cpu(CPUState *cpu, run_on_cpu_data data)
>> +{
>> + CPUAddressSpace *cpuas = data.host_ptr;
>> +
>> + cpuas->memory_dispatch = address_space_to_dispatch(cpuas->as);
>> + tlb_flush(cpu);
>> +}
>
> Question: do I need to take the iothread lock here, while
> re-generating the address space dispatch?
Does it regenerate or just collect a current live version under RCU?
--
Alex Bennée
Virtualisation Tech Lead @ Linaro
On 8/27/23 13:17, Alex Bennée wrote:
>
> Richard Henderson <richard.henderson@linaro.org> writes:
>
>> On 8/26/23 16:24, Richard Henderson wrote:
>>> +static void tcg_commit_cpu(CPUState *cpu, run_on_cpu_data data)
>>> +{
>>> + CPUAddressSpace *cpuas = data.host_ptr;
>>> +
>>> + cpuas->memory_dispatch = address_space_to_dispatch(cpuas->as);
>>> + tlb_flush(cpu);
>>> +}
>>
>> Question: do I need to take the iothread lock here, while
>> re-generating the address space dispatch?
>
> Does it regenerate or just collect a current live version under RCU?
Quite right, just reads.
r~
Richard Henderson <richard.henderson@linaro.org> writes:
> After system startup, run the update to memory_dispatch
> and the tlb_flush on the cpu. This eliminates a race,
> wherein a running cpu sees the memory_dispatch change
> but has not yet seen the tlb_flush.
>
> Since the update now happens on the cpu, we need not use
> qatomic_rcu_read to protect the read of memory_dispatch.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1826
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1834
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1846
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
> ---
> softmmu/physmem.c | 40 +++++++++++++++++++++++++++++-----------
> 1 file changed, 29 insertions(+), 11 deletions(-)
>
> diff --git a/softmmu/physmem.c b/softmmu/physmem.c
> index 7597dc1c39..18277ddd67 100644
> --- a/softmmu/physmem.c
> +++ b/softmmu/physmem.c
> @@ -680,8 +680,7 @@ address_space_translate_for_iotlb(CPUState *cpu, int asidx, hwaddr orig_addr,
> IOMMUTLBEntry iotlb;
> int iommu_idx;
> hwaddr addr = orig_addr;
> - AddressSpaceDispatch *d =
> - qatomic_rcu_read(&cpu->cpu_ases[asidx].memory_dispatch);
> + AddressSpaceDispatch *d = cpu->cpu_ases[asidx].memory_dispatch;
>
> for (;;) {
> section = address_space_translate_internal(d, addr, &addr, plen, false);
> @@ -2412,7 +2411,7 @@ MemoryRegionSection *iotlb_to_section(CPUState *cpu,
> {
> int asidx = cpu_asidx_from_attrs(cpu, attrs);
> CPUAddressSpace *cpuas = &cpu->cpu_ases[asidx];
> - AddressSpaceDispatch *d = qatomic_rcu_read(&cpuas->memory_dispatch);
> + AddressSpaceDispatch *d = cpuas->memory_dispatch;
> int section_index = index & ~TARGET_PAGE_MASK;
> MemoryRegionSection *ret;
>
> @@ -2487,23 +2486,42 @@ static void tcg_log_global_after_sync(MemoryListener *listener)
> }
> }
>
> +static void tcg_commit_cpu(CPUState *cpu, run_on_cpu_data data)
> +{
> + CPUAddressSpace *cpuas = data.host_ptr;
> +
> + cpuas->memory_dispatch = address_space_to_dispatch(cpuas->as);
> + tlb_flush(cpu);
> +}
> +
> static void tcg_commit(MemoryListener *listener)
> {
> CPUAddressSpace *cpuas;
> - AddressSpaceDispatch *d;
> + CPUState *cpu;
>
> assert(tcg_enabled());
> /* since each CPU stores ram addresses in its TLB cache, we must
> reset the modified entries */
> cpuas = container_of(listener, CPUAddressSpace, tcg_as_listener);
> - cpu_reloading_memory_map();
> - /* The CPU and TLB are protected by the iothread lock.
> - * We reload the dispatch pointer now because cpu_reloading_memory_map()
> - * may have split the RCU critical section.
> + cpu = cpuas->cpu;
> +
> + /*
> + * Defer changes to as->memory_dispatch until the cpu is quiescent.
> + * Otherwise we race between (1) other cpu threads and (2) ongoing
> + * i/o for the current cpu thread, with data cached by mmu_lookup().
> + *
> + * In addition, queueing the work function will kick the cpu back to
> + * the main loop, which will end the RCU critical section and reclaim
> + * the memory data structures.
> + *
> + * That said, the listener is also called during realize, before
> + * all of the tcg machinery for run-on is initialized: thus halt_cond.
> */
> - d = address_space_to_dispatch(cpuas->as);
> - qatomic_rcu_set(&cpuas->memory_dispatch, d);
> - tlb_flush(cpuas->cpu);
> + if (cpu->halt_cond) {
> + async_run_on_cpu(cpu, tcg_commit_cpu, RUN_ON_CPU_HOST_PTR(cpuas));
> + } else {
> + tcg_commit_cpu(cpu, RUN_ON_CPU_HOST_PTR(cpuas));
> + }
> }
>
> static void memory_map_init(void)
--
Alex Bennée
Virtualisation Tech Lead @ Linaro
© 2016 - 2026 Red Hat, Inc.