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softmmu: Use async_run_on_cpu in tcg_commit

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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
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2023-08-25 16:13:17 -07:00
parent 86e4f93d82
commit 0d58c66068
3 changed files with 29 additions and 42 deletions
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30
accel/tcg/cpu-exec-common.c
View File

@ -33,36 +33,6 @@ void cpu_loop_exit_noexc(CPUState *cpu)
cpu_loop_exit(cpu);
}
#if defined(CONFIG_SOFTMMU)
void cpu_reloading_memory_map(void)
{
if (qemu_in_vcpu_thread() && current_cpu->running) {
/* The guest can in theory prolong the RCU critical section as long
* as it feels like. The major problem with this is that because it
* can do multiple reconfigurations of the memory map within the
* critical section, we could potentially accumulate an unbounded
* collection of memory data structures awaiting reclamation.
*
* Because the only thing we're currently protecting with RCU is the
* memory data structures, it's sufficient to break the critical section
* in this callback, which we know will get called every time the
* memory map is rearranged.
*
* (If we add anything else in the system that uses RCU to protect
* its data structures, we will need to implement some other mechanism
* to force TCG CPUs to exit the critical section, at which point this
* part of this callback might become unnecessary.)
*
* This pair matches cpu_exec's rcu_read_lock()/rcu_read_unlock(), which
* only protects cpu->as->dispatch. Since we know our caller is about
* to reload it, it's safe to split the critical section.
*/
rcu_read_unlock();
rcu_read_lock();
}
}
#endif
void cpu_loop_exit(CPUState *cpu)
{
/* Undo the setting in cpu_tb_exec. */

1
include/exec/cpu-common.h
View File

@ -133,7 +133,6 @@ static inline void cpu_physical_memory_write(hwaddr addr,
{
cpu_physical_memory_rw(addr, (void *)buf, len, true);
}
void cpu_reloading_memory_map(void);
void *cpu_physical_memory_map(hwaddr addr,
hwaddr *plen,
bool is_write);

40
softmmu/physmem.c
View File

@ -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)