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Diffstat (limited to 'results/classifier/105/other/74715356')
| -rw-r--r-- | results/classifier/105/other/74715356 | 134 |
1 files changed, 134 insertions, 0 deletions
diff --git a/results/classifier/105/other/74715356 b/results/classifier/105/other/74715356 new file mode 100644 index 000000000..8725e032f --- /dev/null +++ b/results/classifier/105/other/74715356 @@ -0,0 +1,134 @@ +other: 0.927 +semantic: 0.916 +instruction: 0.910 +assembly: 0.905 +device: 0.900 +graphic: 0.894 +boot: 0.881 +mistranslation: 0.870 +KVM: 0.863 +vnc: 0.850 +socket: 0.843 +network: 0.838 + +[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: +   +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. + +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 implementation +     +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. +In my case, after loop reentry, it led to an invalid branch to be taken. +Thanks for your time! +Regards +Stevie + +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 + |