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[Bug] x86 EFLAGS refresh is not happening correctly
Hello,
I'm posting this here instead of opening an issue as it is not clear to me if this is a bug or not.
The issue is located in function "cpu_compute_eflags" in target/i386/cpu.h
(
https://gitlab.com/qemu-project/qemu/-/blob/master/target/i386/cpu.h#L2071
)
This function is exectued in an out of cpu loop context.
It is used to synchronize TCG internal eflags registers (CC_OP, CC_SRC, etc...) with the CPU eflags field upon loop exit.
It does:
  eflags
|=
cpu_cc_compute_all
(
env
,
CC_OP
)
|
(
env
->
df
&
DF_MASK
);
Shouldn't it be:
  Â
eflags
=
cpu_cc_compute_all
(
env
,
CC_OP
)
|
(
env
->
df
&
DF_MASK
);
as eflags is entirely reevaluated by "cpu_cc_compute_all" ?
Thanks,
Kind regards,
Stevie
On 05/08/21 11:51, Stevie Lavern wrote:
Shouldn't it be:
eflags = cpu_cc_compute_all(env, CC_OP) | (env->df & DF_MASK);
as eflags is entirely reevaluated by "cpu_cc_compute_all" ?
No, both are wrong. env->eflags contains flags other than the
arithmetic flags (OF/SF/ZF/AF/PF/CF) and those have to be preserved.
The right code is in helper_read_eflags. You can move it into
cpu_compute_eflags, and make helper_read_eflags use it.
Paolo
On 05/08/21 13:24, Paolo Bonzini wrote:
On 05/08/21 11:51, Stevie Lavern wrote:
Shouldn't it be:
eflags = cpu_cc_compute_all(env, CC_OP) | (env->df & DF_MASK);
as eflags is entirely reevaluated by "cpu_cc_compute_all" ?
No, both are wrong. env->eflags contains flags other than the
arithmetic flags (OF/SF/ZF/AF/PF/CF) and those have to be preserved.
The right code is in helper_read_eflags. You can move it into
cpu_compute_eflags, and make helper_read_eflags use it.
Ah, actually the two are really the same, the TF/VM bits do not apply to
cpu_compute_eflags so it's correct.
What seems wrong is migration of the EFLAGS register. There should be
code in cpu_pre_save and cpu_post_load to special-case it and setup
CC_DST/CC_OP as done in cpu_load_eflags.
Also, cpu_load_eflags should assert that update_mask does not include
any of the arithmetic flags.
Paolo
Thank for your reply!
It's still a bit cryptic for me.
I think i need to precise that I'm using a x86_64 custom user-mode,base on linux user-mode, that i'm developing (unfortunately i cannot share the code) with modifications in the translation loop (I've added cpu loop exits on specific instructions which are not control flow instructions).
If my understanding is correct, in the user-mode case 'cpu_compute_eflags' is called directly by 'x86_cpu_exec_exit' with the intention of synchronizing the CPU env->eflags field with its real value (represented by the CC_* fields).
I'm not sure how 'cpu_pre_save' and 'cpu_post_load' are involved in this case.
Â
As you said in your first email, 'helper_read_eflags' seems to be the correct way to go.
Here is some detail about my current experimentation/understanding of this "issue":
With the current implementationÂ
    Â
eflags |= cpu_cc_compute_all(env, CC_OP) | (env->df & DF_MASK);
if I exit the loop with a CC_OP different from CC_OP_EFLAGS, I found that the resulting env->eflags may be invalid.
In my test case, the loop was exiting with eflags = 0x44 and CC_OP = CC_OP_SUBL with CC_DST=1, CC_SRC=258, CC_SRC2=0.
While 'cpu_cc_compute_all' computes the correct flags (ZF:0, PF:0), the result will still be 0x44 (ZF:1, PF:1) due to the 'or' operation, thus leading to an incorrect eflags value loaded into the CPU env.Â
In my case, after loop reentry, it led to an invalid branch to be taken.
Thanks for your time!
Regards
Stevie
Â
On Thu, Aug 5, 2021 at 1:33 PM Paolo Bonzini <
pbonzini@redhat.com
> wrote:
On 05/08/21 13:24, Paolo Bonzini wrote:
> On 05/08/21 11:51, Stevie Lavern wrote:
>>
>> Shouldn't it be:
>> eflags = cpu_cc_compute_all(env, CC_OP) | (env->df & DF_MASK);
>> as eflags is entirely reevaluated by "cpu_cc_compute_all" ?
>
> No, both are wrong. env->eflags contains flags other than the
> arithmetic flags (OF/SF/ZF/AF/PF/CF) and those have to be preserved.
>
> The right code is in helper_read_eflags. You can move it into
> cpu_compute_eflags, and make helper_read_eflags use it.
Ah, actually the two are really the same, the TF/VM bits do not apply to
cpu_compute_eflags so it's correct.
What seems wrong is migration of the EFLAGS register. There should be
code in cpu_pre_save and cpu_post_load to special-case it and setup
CC_DST/CC_OP as done in cpu_load_eflags.
Also, cpu_load_eflags should assert that update_mask does not include
any of the arithmetic flags.
Paolo
|