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Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire
From: |
Pavel Dovgalyuk |
Subject: |
Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire |
Date: |
Thu, 26 Jan 2017 16:37:40 +0300 |
> From: Paolo Bonzini [mailto:address@hidden On Behalf Of Paolo Bonzini
> On 26/01/2017 13:34, Pavel Dovgalyuk wrote:
> > This patch adds check to break cpu loop when icount expires without
> > setting the TB_EXIT_ICOUNT_EXPIRED flag. It happens when there is no
> > available translated blocks and all instructions were executed.
> > In icount replay mode unnecessary tb_find will be called (which may
> > cause an exception) and execution will be non-deterministic.
> >
> > v8: refactored loop exit code and moved it to separate function
> >
> > Signed-off-by: Pavel Dovgalyuk <address@hidden>
> > ---
> > cpu-exec.c | 24 +++++++++++++++++++-----
> > 1 file changed, 19 insertions(+), 5 deletions(-)
> >
> > diff --git a/cpu-exec.c b/cpu-exec.c
> > index fa08c73..f9b8ec8 100644
> > --- a/cpu-exec.c
> > +++ b/cpu-exec.c
> > @@ -523,9 +523,25 @@ static inline void cpu_handle_interrupt(CPUState *cpu,
> > *last_tb = NULL;
> > }
> > }
> > - if (unlikely(atomic_read(&cpu->exit_request) ||
> > replay_has_interrupt())) {
> > +}
> > +
> > +
> > +static void cpu_check_loop_exit(CPUState *cpu)
> > +{
> > + if (unlikely(atomic_read(&cpu->exit_request)
> > + /* icount has expired, we need to break the execution loop.
> > + This check is needed before tb_find to make execution
> > + deterministic - tb_find may cause an exception
> > + while translating the code from non-mapped page. */
> > + || (use_icount && ((cpu->icount_extra == 0
> > + && cpu->icount_decr.u16.low == 0)
> > + || (int32_t)cpu->icount_decr.u32 < 0)))) {
>
> Simpler:
>
> use_icount &&
> ((int32_t)cpu->icount_decr.u32 < 0 ||
> cpu->icount_decr.low + cpu->icount_extra == 0)
Right.
> But I'm not sure that you need to test u32. After all you're not
Checking u32 is needed, because sometimes it is less than zero.
Consider the code in cpu_loop_exec_tb:
int insns_left = cpu->icount_decr.u32;
if (insns_left > 0) {
It is set to negative in tcg_handle_interrupt.
Ten days ago you also offered setting high bits of u32 in gen_icount:
+ /* Make the next TB exit immediately with TB_EXIT_ICOUNT_EXPIRED. */
+ tcg_gen_st16_i32(-1, cpu_env,
+ -ENV_OFFSET + offsetof(CPUState, icount_decr.u16.high));
> testing tcg_exit_req, which is the equivalent when icount is disabled.
Yes, if we want to refactor the whole loop.
> > atomic_set(&cpu->exit_request, 0);
> > - cpu->exception_index = EXCP_INTERRUPT;
> > + /* If there is an exception that wasn't replayed yet,
> > + don't change exception_index. */
> > + if (cpu->exception_index == -1) {
> > + cpu->exception_index = EXCP_INTERRUPT;
> > + }
> > cpu_loop_exit(cpu);
>
> The siglongjmp is effectively the same as exiting the for(;;) loop of
> cpu_exec and going back to cpu_handle_exception. So I would just merge
> this with cpu_handle_interrupt, which exits a lot with cpu_loop_exit too.
Do you want me to create new version of the patch, which changes the loop this
way?
> All cpu_loop_exit() calls in cpu_handle_interrupt become "return true",
> similar to cpu_handle_exception, and then in cpu_exec you have:
>
> /* if an exception is pending, we execute it here */
> while (!cpu_handle_exception(cpu, &ret)) {
> /* if an interrupt is pending, inject it and go back
> * to cpu_handle_exception.
> */
> while (!cpu_handle_interrupt(cpu, &last_tb)) {
> tb = tb_find(cpu, last_tb, tb_exit);
> cpu_loop_exec_tb(cpu, tb, &last_tb, &tb_exit, &sc);
> /* Try to align the host and virtual clocks
> if the guest is in advance */
> align_clocks(&sc, cpu);
> }
> }
> break;
It might even work faster without excessive cpu_loop_exit calls.
Pavel Dovgalyuk
- [Qemu-devel] [PATCH v8 0/9] replay additions, Pavel Dovgalyuk, 2017/01/26
- [Qemu-devel] [PATCH v8 1/9] replay: exception replay fix, Pavel Dovgalyuk, 2017/01/26
- [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire, Pavel Dovgalyuk, 2017/01/26
- Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire, Paolo Bonzini, 2017/01/26
- Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire,
Pavel Dovgalyuk <=
- Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire, Paolo Bonzini, 2017/01/26
- Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire, Pavel Dovgalyuk, 2017/01/26
- Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire, Paolo Bonzini, 2017/01/26
- Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire, Pavel Dovgalyuk, 2017/01/27
- Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire, Paolo Bonzini, 2017/01/27
[Qemu-devel] [PATCH v8 3/9] apic: save apic_delivered flag, Pavel Dovgalyuk, 2017/01/26