Re: Crash with CXL + TCG on 8.2: Was Re: qemu cxl memory expander shows numa_node -1

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

Jonathan Cameron <Jonathan.Cameron@huawei.com> writes:

> On Thu, 1 Feb 2024 16:00:56 +0000
> Peter Maydell <peter.maydell@linaro.org> wrote:
>
>> On Thu, 1 Feb 2024 at 15:17, Alex Bennée <alex.bennee@linaro.org> wrote:
>> >
>> > Peter Maydell <peter.maydell@linaro.org> writes:  
>> > > So, that looks like:
>> > >  * we call cpu_tb_exec(), which executes some generated code
>> > >  * that generated code calls the lookup_tb_ptr helper to see
>> > >    if we have a generated TB already for the address we're going
>> > >    to execute next
>> > >  * lookup_tb_ptr probes the TLB to see if we know the host RAM
>> > >    address for the guest address
>> > >  * this results in a TLB walk for an instruction fetch
>> > >  * the page table descriptor load is to IO memory
>> > >  * io_prepare assumes it needs to do a TLB recompile, because
>> > >    can_do_io is clear
>> > >
>> > > I am not surprised that the corner case of "the guest put its
>> > > page tables in an MMIO device" has not yet come up :-)
>> > >
>> > > I'm really not sure how the icount handling should interact
>> > > with that...  
>> >
>> > Its not just icount - we need to handle it for all modes now. That said
>> > seeing as we are at the end of a block shouldn't can_do_io be set?  
>> 
>> The lookup_tb_ptr helper gets called from tcg_gen_goto_tb(),
>> which happens earlier than the tb_stop callback (it can
>> happen in the trans function for branch etc insns, for
>> example).
>> 
>> I think it should be OK to clear can_do_io at the start
>> of the lookup_tb_ptr helper, something like:
>> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
>> index 977576ca143..7818537f318 100644
>> --- a/accel/tcg/cpu-exec.c
>> +++ b/accel/tcg/cpu-exec.c
>> @@ -396,6 +396,15 @@ const void *HELPER(lookup_tb_ptr)(CPUArchState *env)
>>      uint64_t cs_base;
>>      uint32_t flags, cflags;
>> 
>> +    /*
>> +     * By definition we've just finished a TB, so I/O is OK.
>> +     * Avoid the possibility of calling cpu_io_recompile() if
>> +     * a page table walk triggered by tb_lookup() calling
>> +     * probe_access_internal() happens to touch an MMIO device.
>> +     * The next TB, if we chain to it, will clear the flag again.
>> +     */
>> +    cpu->neg.can_do_io = true;
>> +
>>      cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
>> 
>>      cflags = curr_cflags(cpu);
>> 
>> -- PMM
>
> No joy.  Seems like a very similar backtrace.
>
> Thread 5 "qemu-system-x86" received signal SIGABRT, Aborted.
> [Switching to Thread 0x7ffff4efe6c0 (LWP 23937)]
> __pthread_kill_implementation (no_tid=0, signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:44
> 44      ./nptl/pthread_kill.c: No such file or directory.
> (gdb) bt
> #0  __pthread_kill_implementation (no_tid=0, signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:44
> #1  __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78
> #2  __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:89
> #3  0x00007ffff77c43b6 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
> #4  0x00007ffff77aa87c in __GI_abort () at ./stdlib/abort.c:79
> #5  0x0000555555c4d19e in cpu_abort (cpu=cpu@entry=0x5555578e0cb0, fmt=fmt@entry=0x555556048ee8 "cpu_io_recompile: could not find TB for pc=%p") at ../../cpu-target.c:373
> #6  0x0000555555c9cb25 in cpu_io_recompile (cpu=cpu@entry=0x5555578e0cb0, retaddr=retaddr@entry=0) at ../../accel/tcg/translate-all.c:611
> #7  0x0000555555c9f744 in io_prepare (retaddr=0, addr=19595790664, attrs=..., xlat=<optimized out>, cpu=0x5555578e0cb0, out_offset=<synthetic pointer>) at ../../accel/tcg/cputlb.c:1339
> #8  do_ld_mmio_beN (cpu=0x5555578e0cb0, full=0x7ffe88012890, ret_be=ret_be@entry=0, addr=19595790664, size=size@entry=8, mmu_idx=4, type=MMU_DATA_LOAD, ra=0) at ../../accel/tcg/cputlb.c:2030
> #9  0x0000555555ca0ecd in do_ld_8 (cpu=cpu@entry=0x5555578e0cb0, p=p@entry=0x7ffff4efcdd0, mmu_idx=<optimized out>, type=type@entry=MMU_DATA_LOAD, memop=<optimized out>, ra=ra@entry=0) at ../../accel/tcg/cputlb.c:2356
> #10 0x0000555555ca332f in do_ld8_mmu (cpu=cpu@entry=0x5555578e0cb0, addr=addr@entry=19595790664, oi=oi@entry=52, ra=ra@entry=0, access_type=access_type@entry=MMU_DATA_LOAD) at ../../accel/tcg/cputlb.c:2439
> #11 0x0000555555ca5e69 in cpu_ldq_mmu (ra=0, oi=52, addr=19595790664, env=0x5555578e3470) at ../../accel/tcg/ldst_common.c.inc:169
> #12 cpu_ldq_le_mmuidx_ra (env=0x5555578e3470, addr=19595790664, mmu_idx=<optimized out>, ra=ra@entry=0) at ../../accel/tcg/ldst_common.c.inc:301
> #13 0x0000555555b4b5de in ptw_ldq (in=0x7ffff4efcf10) at ../../target/i386/tcg/sysemu/excp_helper.c:98
> #14 ptw_ldq (in=0x7ffff4efcf10) at ../../target/i386/tcg/sysemu/excp_helper.c:93
> #15 mmu_translate (env=env@entry=0x5555578e3470, in=0x7ffff4efcfd0, out=0x7ffff4efcfa0, err=err@entry=0x7ffff4efcfb0) at ../../target/i386/tcg/sysemu/excp_helper.c:173
> #16 0x0000555555b4c3f3 in get_physical_address (err=0x7ffff4efcfb0, out=0x7ffff4efcfa0, mmu_idx=0, access_type=MMU_DATA_STORE, addr=18386491786698339392, env=0x5555578e3470) at ../../target/i386/tcg/sysemu/excp_helper.c:578
> #17 x86_cpu_tlb_fill (cs=0x5555578e0cb0, addr=18386491786698339392, size=<optimized out>, access_type=MMU_DATA_STORE, mmu_idx=0, probe=<optimized out>, retaddr=140736029817822) at ../../target/i386/tcg/sysemu/excp_helper.c:604
> #18 0x0000555555ca0df9 in tlb_fill (retaddr=140736029817822, mmu_idx=0, access_type=MMU_DATA_STORE, size=<optimized out>, addr=18386491786698339392, cpu=0x7ffff4efd120) at ../../accel/tcg/cputlb.c:1315
> #19 mmu_lookup1 (cpu=cpu@entry=0x5555578e0cb0, data=data@entry=0x7ffff4efd120, mmu_idx=0, access_type=access_type@entry=MMU_DATA_STORE, ra=ra@entry=140736029817822) at ../../accel/tcg/cputlb.c:1713
> #20 0x0000555555ca2b71 in mmu_lookup (cpu=0x5555578e0cb0, addr=18386491786698339392, oi=<optimized out>, ra=140736029817822, type=MMU_DATA_STORE, l=0x7ffff4efd120) at ../../accel/tcg/cputlb.c:1803
> #21 0x0000555555ca3e5d in do_st8_mmu (cpu=0x5555578e0cb0, addr=23937, val=18386491784638059520, oi=6, ra=140736029817822) at ../../accel/tcg/cputlb.c:2853
> #22 0x00007fffa9107c63 in code_gen_buffer ()

No thats different - we are actually writing to the MMIO region here.
But the fact we hit cpu_abort because we can't find the TB we are
executing is a little problematic.

Does ra properly point to the code buffer here?

> #23 0x0000555555c9395b in cpu_tb_exec (cpu=cpu@entry=0x5555578e0cb0, itb=itb@entry=0x7fffa9107980 <code_gen_buffer+17856851>, tb_exit=tb_exit@entry=0x7ffff4efd718) at ../../accel/tcg/cpu-exec.c:442
> #24 0x0000555555c93ec0 in cpu_loop_exec_tb (tb_exit=0x7ffff4efd718, last_tb=<synthetic pointer>, pc=<optimized out>, tb=0x7fffa9107980 <code_gen_buffer+17856851>, cpu=0x5555578e0cb0) at ../../accel/tcg/cpu-exec.c:897
> #25 cpu_exec_loop (cpu=cpu@entry=0x5555578e0cb0, sc=sc@entry=0x7ffff4efd7b0) at ../../accel/tcg/cpu-exec.c:1012
> #26 0x0000555555c946d1 in cpu_exec_setjmp (cpu=cpu@entry=0x5555578e0cb0, sc=sc@entry=0x7ffff4efd7b0) at ../../accel/tcg/cpu-exec.c:1029
> #27 0x0000555555c94ebc in cpu_exec (cpu=cpu@entry=0x5555578e0cb0) at ../../accel/tcg/cpu-exec.c:1055
> #28 0x0000555555cb8f53 in tcg_cpu_exec (cpu=cpu@entry=0x5555578e0cb0) at ../../accel/tcg/tcg-accel-ops.c:76
> #29 0x0000555555cb90b0 in mttcg_cpu_thread_fn (arg=arg@entry=0x5555578e0cb0) at ../../accel/tcg/tcg-accel-ops-mttcg.c:95
> #30 0x0000555555e57180 in qemu_thread_start (args=0x555557956000) at ../../util/qemu-thread-posix.c:541
> #31 0x00007ffff78176ba in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:444
> #32 0x00007ffff78a60d0 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro