RE: [PATCH v6 27/33] x86/fred: fixup fault on ERETU by jumping to fred_entrypoint_user

[Li, Xin3]

> > +       /* Copy error code to uregs and adjust stack pointer accordingly */
> > +       uregs->orig_ax = error_code;
> 
> The address of uregs->orig_ax is below regs->sp, so I think some comments are
> needed here to state why it is safe to write to uregs->orig_ax (a.k.a it is not
> verlapped with regs).

Good point, because it's one of the nice FRED features.

The RSP used by FRED to push a stack frame is not the value in %rsp, it is
calculated from %rsp with the following 2 steps:
  1) RSP = %rsp - (IA32_FRED_CONFIG & 0x1c0) // REDZONE of (N * 64) bytes
  2) RSP = RSP & ~0x3f // Clearing RSP[5:0] to align to a 64-byte cache line
when the event delivery doesn't trigger a stack level change.

Thus the FRED stack frame error code, i.e., orig_ax, is _always_ on a 64-byte
cache line boundary, and a new stack frame is guaranteed to start below the
error code (An extra REDZONE of (N * 64) bytes may be pushed between), and
it is safe to write to uregs->orig_ax.

Here is an example with a N=1 REDZONE:

  64-byte cache line ==>  ______________
                         |___Reserved___|
                         |__Event_data__|
                         |_____SS_______|
                         |_____RSP______|
                         |_____FLAGS____|
                         |_____CS_______|
                         |_____IP_______|  <== ERETU stack frame
  64-byte cache line ==> |__Error_code__|
                         |______________|
                         |______________|
                         |______________|
                         |______________|
                         |______________|
                         |______________|
                         |______________|  <== RSP after step 1)
  64-byte cache line ==> |______________|  <== RSP after step 2)
                         |___Reserved___|
                         |__Event_data__|
                         |_____SS_______|
                         |_____RSP______|
                         |_____FLAGS____|
                         |_____CS_______|
                         |_____IP_______|  <== ERETS stack frame
  64-byte cache line ==> |__Error_code__|

    Xin