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Date: Fri, 23 Dec 2016 19:33:03 +0000
From: Doug Evans <dje@google.com>
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Subject: Re: [Qemu-devel] [PULL 24/25] target-i386: Fix eflags.TF/#DB
 handling of syscall/sysret insns
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org

Paolo Bonzini writes:
  > From: Doug Evans <dje@google.com>
  >
  > The syscall and sysret instructions behave a bit differently:
  > TF is checked after the instruction completes.
  > This allows the o/s to disable #DB at a syscall by adding TF to FMASK.
  > And then when the sysret is executed the #DB is taken "as if" the
  > syscall insn just completed.
  >
  > Signed-off-by: Doug Evans <dje@google.com>
  > Message-Id: <94eb2c0bfa1c6a9fec0543057483@google.com>
  > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
  > ---
  >  target/i386/bpt_helper.c |  7 +++++++
  >  target/i386/helper.h     |  1 +
  >  target/i386/translate.c  | 29 ++++++++++++++++++++++++-----
  >  3 files changed, 32 insertions(+), 5 deletions(-)
  >
  > ...
  > diff --git a/target/i386/translate.c b/target/i386/translate.c
  > index 324103c..59e11fc 100644
  > --- a/target/i386/translate.c
  > +++ b/target/i386/translate.c
  > @@ -6423,7 +6435,10 @@ static target_ulong disas_insn(CPUX86State *env,  
DisasContext *s,
  >                                        tcg_const_i32(s->pc -  
s->cs_base));
  >              set_cc_op(s, CC_OP_EFLAGS);
  >          }
  > -        gen_eob(s);
  > +        /* TF handling for the syscall insn is different. The TF bit is  
checked
  > +           after the syscall insn completes. This allows #DB to not be
  > +           generated after one has entered CPL0 if TF is set in FMASK.   
*/
  > +        gen_eob_worker(s, false, true);
  >          break;
  >      case 0xe8: /* call im */
  >          {
  > @@ -7115,7 +7130,11 @@ static target_ulong disas_insn(CPUX86State *env,  
DisasContext *s,
  >              if (s->lma) {
  >                  set_cc_op(s, CC_OP_EFLAGS);
  >              }
  > -            gen_eob(s);
  > +            /* TF handling for the sysret insn is different. The TF bit  
is
  > +               checked after the sysret insn completes. This allows #DB  
to be
  > +               generated "as if" the syscall insn in userspace has just
  > +               completed.  */
  > +            gen_eob_worker(s, false, true);
  >          }
  >          break;
  >  #endif

Hi. Just a heads up that this patch got applied wrong (patch does that some  
times).

The change to the "syscall" insn got applied to the "iret" instruction  
instead.

Working on a fix.