Re: [PATCH v2 3/6] cputlb: ensure we save the IOTLB data in case of reset

["Alex Bennée" <alex.bennee@linaro.org>]

Emilio G. Cota <cota@braap.org> writes:

> On Wed, Jun 10, 2020 at 16:55:06 +0100, Alex Bennée wrote:
>> Any write to a device might cause a re-arrangement of memory
>> triggering a TLB flush and potential re-size of the TLB invalidating
>> previous entries. This would cause users of qemu_plugin_get_hwaddr()
>> to see the warning:
>> 
>>   invalid use of qemu_plugin_get_hwaddr
>> 
>> because of the failed tlb_lookup which should always succeed. To
>> prevent this we save the IOTLB data in case it is later needed by a
>> plugin doing a lookup.
>> 
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> 
>> ---
>> v2
>>   - save the entry instead of re-running the tlb_fill.
>> 
>> squash! cputlb: ensure we save the IOTLB entry in case of reset
>> ---
>>  accel/tcg/cputlb.c | 63 ++++++++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 61 insertions(+), 2 deletions(-)
>> 
>> diff --git a/accel/tcg/cputlb.c b/accel/tcg/cputlb.c
>> index eb2cf9de5e6..9bf9e479c7c 100644
>> --- a/accel/tcg/cputlb.c
>> +++ b/accel/tcg/cputlb.c
>> @@ -1058,6 +1058,47 @@ static uint64_t io_readx(CPUArchState *env, CPUIOTLBEntry *iotlbentry,
>>      return val;
>>  }
>>  
>> +#ifdef CONFIG_PLUGIN
>> +
>> +typedef struct SavedIOTLB {
>> +    struct rcu_head rcu;
>> +    struct SavedIOTLB **save_loc;
>> +    MemoryRegionSection *section;
>> +    hwaddr mr_offset;
>> +} SavedIOTLB;
>> +
>> +static void clean_saved_entry(SavedIOTLB *s)
>> +{
>> +    atomic_rcu_set(s->save_loc, NULL);
>
> This will race with the CPU thread that sets saved_for_plugin in
> save_iotlb_data().

Surely that only happens outside the critical section?

>
>> +    g_free(s);
>> +}
>> +
>> +static __thread SavedIOTLB *saved_for_plugin;
>
> Apologies if this has been discussed, but why is this using TLS
> variables and not state embedded in CPUState?

Good point - I guess I;m being lazy.

> I see that qemu_plugin_get_hwaddr does not take a cpu_index, but
> maybe it should? We could then just embed the RCU pointer in CPUState.
>
>> +
>> +/*
>> + * Save a potentially trashed IOTLB entry for later lookup by plugin.
>> + *
>> + * We also need to track the thread storage address because the RCU
>> + * cleanup that runs when we leave the critical region (the current
>> + * execution) is actually in a different thread.
>> + */
>> +static void save_iotlb_data(MemoryRegionSection *section, hwaddr mr_offset)
>> +{
>> +    SavedIOTLB *s = g_new(SavedIOTLB, 1);
>> +    s->save_loc = &saved_for_plugin;
>> +    s->section = section;
>> +    s->mr_offset = mr_offset;
>> +    atomic_rcu_set(&saved_for_plugin, s);
>> +    call_rcu(s, clean_saved_entry, rcu);
>
> Here we could just publish the new pointer and g_free_rcu the old
> one, if any.

That would be simpler. I'll re-spin.

>
>> +}
>> +
>> +#else
>> +static void save_iotlb_data(MemoryRegionSection *section, hwaddr mr_offset)
>> +{
>> +    /* do nothing */
>> +}
>> +#endif
>> +
>>  static void io_writex(CPUArchState *env, CPUIOTLBEntry *iotlbentry,
>>                        int mmu_idx, uint64_t val, target_ulong addr,
>>                        uintptr_t retaddr, MemOp op)
>> @@ -1077,6 +1118,12 @@ static void io_writex(CPUArchState *env, CPUIOTLBEntry *iotlbentry,
>>      }
>>      cpu->mem_io_pc = retaddr;
>>  
>> +    /*
>> +     * The memory_region_dispatch may trigger a flush/resize
>> +     * so for plugins we save the iotlb_data just in case.
>> +     */
>> +    save_iotlb_data(section, mr_offset);
>> +
>>      if (mr->global_locking && !qemu_mutex_iothread_locked()) {
>>          qemu_mutex_lock_iothread();
>>          locked = true;
>> @@ -1091,6 +1138,7 @@ static void io_writex(CPUArchState *env, CPUIOTLBEntry *iotlbentry,
>>                                 MMU_DATA_STORE, mmu_idx, iotlbentry->attrs, r,
>>                                 retaddr);
>>      }
>> +
>
> Stray whitespace change.
>
>>      if (locked) {
>>          qemu_mutex_unlock_iothread();
>>      }
>> @@ -1366,8 +1414,11 @@ void *tlb_vaddr_to_host(CPUArchState *env, abi_ptr addr,
>>   * in the softmmu lookup code (or helper). We don't handle re-fills or
>>   * checking the victim table. This is purely informational.
>>   *
>> - * This should never fail as the memory access being instrumented
>> - * should have just filled the TLB.
>> + * This almost never fails as the memory access being instrumented
>> + * should have just filled the TLB. The one corner case is io_writex
>> + * which can cause TLB flushes and potential resizing of the TLBs
>> + * loosing the information we need. In those cases we need to recover
>> + * data from a thread local copy of the io_tlb entry.
>>   */
>>  
>>  bool tlb_plugin_lookup(CPUState *cpu, target_ulong addr, int mmu_idx,
>> @@ -1391,6 +1442,14 @@ bool tlb_plugin_lookup(CPUState *cpu, target_ulong addr, int mmu_idx,
>>              data->v.ram.hostaddr = addr + tlbe->addend;
>>          }
>>          return true;
>> +    } else {
>> +        SavedIOTLB *saved = atomic_rcu_read(&saved_for_plugin);
>> +        if (saved) {
>> +            data->is_io = true;
>> +            data->v.io.section = saved->section;
>> +            data->v.io.offset = saved->mr_offset;
>> +            return true;
>> +        }
>
> Shouldn't we check that the contents of the saved IOTLB match the
> parameters of the lookup? Otherwise passing a random address is likely
> to land here.

Good point - I'm being too trusting here ;-)

Thanks for the review.

-- 
Alex Bennée