Re: [Qemu-devel] [PATCH v2 0/9] tcg: signal-free qemu_cpu_kick

[Richard Henderson <rth@twiddle.net>]

On 08/25/2015 05:17 PM, Paolo Bonzini wrote:
> This version of the signal-free qemu_cpu_kick patches is, ehm, much
> better.  Variable are accessed either with Java-style volatiles or
> protected by memory barriers, and the cleanups go further by removing
> qemu/tls.h and C volatiles.
>
> The logic is relatively simple.  The I/O thread does (letters in
> parentheses indicates the synchronizes-with edges):
>
>      run_on_cpu or similar
>      ...
>      seq_cst write 1 to exit_request        (C)
>      seq_cst read tcg_current_cpu to cpu    (B)
>      if not NULL
>         write 1 to cpu->exit_request
>         release barrier                     (A)
>         write 1 to cpu->tcg_exit_req
>
> The CPU thread does either this:
>
>      (in generated code) read cpu->tcg_exit_req
>      acquire barrier                        (A)
>      read cpu->exit_request
>      exit from cpu_exec
>      seq_cst write 0 to exit_request
>      ...
>      flush_queued_work or similar
>
> or this:
>
>      seq_cst write to tcg_current_cpu       (B)
>      seq_cst read from exit_request         (C)
>      exit from cpu_exec
>      seq_cst write 0 to exit_request
>      ...
>      flush_queued_work or similar
>
> The non-TLS tcg_current_cpu will go away with multi-threaded TCG.
>
> Paolo
>
> Paolo Bonzini (9):
>    i8257: rewrite DMA_schedule to avoid hooking into the CPU loop
>    i8257: remove cpu_request_exit irq
>    tcg: introduce tcg_current_cpu
>    remove qemu/tls.h
>    tcg: assign cpu->current_tb in a simpler place
>    tcg: synchronize cpu->exit_request and cpu->tcg_exit_req accesses
>    tcg: synchronize exit_request and tcg_current_cpu accesses
>    use qemu_cpu_kick instead of cpu_exit or qemu_cpu_kick_thread
>    tcg: signal-free qemu_cpu_kick

Reviewed-by: Richard Henderson <rth@twiddle.net>


r~