Hi Mike,

I just did a test with retfq, it looks the compiler will take 'retfq' as a label with a warnig:
"CpuAsm.iii:26: warning: label alone on a line without a colon might be in error"

CpuAsm.nasm is the source file.
Both CpuAsm.iii and CpuAsm.obj are from the build folder.
CpuAsm_dasm.txt is the dasm file using objdump.

I am using "NASM version 2.12.01rc2 compiled on Mar  7 2016".
BTW, if using "O64 retf" instead of "retfq", the generated obj matches with DB one.
Do you prefer using " O64 retf" comparing with "DB 0x48, 0xcb"?

Thanks,
Guo

> -----Original Message-----
> From: Kinney, Michael D <michael.d.kinney@intel.com>
> Sent: Tuesday, January 5, 2021 7:01 PM
> To: Dong, Guo <guo.dong@intel.com>; devel@edk2.groups.io; Kinney, Michael
> D <michael.d.kinney@intel.com>
> Cc: Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>;
> lersek@redhat.com; Kumar, Rahul1 <rahul1.kumar@intel.com>
> Subject: RE: [edk2-devel] [PATCH] UefiCpuPkg/CpuDxe: Fix boot error
> 
> Hi Guo,
> 
> Could be the port from MASM to NASM did not check to see if NASM supported
> the instruction.
> 
> You can verify the NASM disassembly to make sure it matches the DB bytes.
> 
> Mike
> 
> > -----Original Message-----
> > From: Dong, Guo <guo.dong@intel.com>
> > Sent: Tuesday, January 5, 2021 4:51 PM
> > To: Kinney, Michael D <michael.d.kinney@intel.com>; devel@edk2.groups.io
> > Cc: Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>;
> lersek@redhat.com; Kumar, Rahul1
> > <rahul1.kumar@intel.com>
> > Subject: RE: [edk2-devel] [PATCH] UefiCpuPkg/CpuDxe: Fix boot error
> >
> >
> > Hi Mike,
> >
> > Thanks for the comments. I will remove DB and submit a new patch.
> > I used DB because retfq is used in EDK2 only in
> OvmfPkg\Library\LoadLinuxLib\X64\JumpToKernel.nasm and it used DB.
> > Not sure if there is any BKM why they use it.
> >
> > Thanks,
> > Guo
> >
> > > -----Original Message-----
> > > From: Kinney, Michael D <michael.d.kinney@intel.com>
> > > Sent: Monday, January 4, 2021 9:31 PM
> > > To: devel@edk2.groups.io; Dong, Guo <guo.dong@intel.com>; Kinney,
> Michael
> > > D <michael.d.kinney@intel.com>
> > > Cc: Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>;
> > > lersek@redhat.com; Kumar, Rahul1 <rahul1.kumar@intel.com>
> > > Subject: RE: [edk2-devel] [PATCH] UefiCpuPkg/CpuDxe: Fix boot error
> > >
> > > Hi Guo,
> > >
> > > NASM has good support for instructions.  Can the DB be removed and
> replaced
> > > with the equivalent instruction?
> > >
> > > Thanks,
> > >
> > > Mike
> > >
> > > > -----Original Message-----
> > > > From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Guo
> Dong
> > > > Sent: Thursday, December 24, 2020 12:04 PM
> > > > To: devel@edk2.groups.io
> > > > Cc: Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>;
> > > lersek@redhat.com; Kumar, Rahul1
> > > > <rahul1.kumar@intel.com>
> > > > Subject: [edk2-devel] [PATCH] UefiCpuPkg/CpuDxe: Fix boot error
> > > >
> > > > REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3084
> > > >
> > > > When DXE drivers are dispatched above 4GB memory in 64bit
> > > > mode, the address setCodeSelectorLongJump in stack will
> > > > be override by parameter. Jump to Qword is not supported
> > > > by some processors. So use retfq instead.
> > > >
> > > > Signed-off-by: Guo Dong <guo.dong@intel.com>
> > > > ---
> > > >  UefiCpuPkg/CpuDxe/X64/CpuAsm.nasm | 8 +++-----
> > > >  1 file changed, 3 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/UefiCpuPkg/CpuDxe/X64/CpuAsm.nasm
> > > b/UefiCpuPkg/CpuDxe/X64/CpuAsm.nasm
> > > > index c3489bcc3e..e33ddb2784 100644
> > > > --- a/UefiCpuPkg/CpuDxe/X64/CpuAsm.nasm
> > > > +++ b/UefiCpuPkg/CpuDxe/X64/CpuAsm.nasm
> > > > @@ -20,13 +20,11 @@
> > > >  ;------------------------------------------------------------------------------
> > > >  global ASM_PFX(SetCodeSelector)
> > > >  ASM_PFX(SetCodeSelector):
> > > > -    sub     rsp, 0x10
> > > > +    push    rcx,
> > > >      lea     rax, [setCodeSelectorLongJump]
> > > > -    mov     [rsp], rax
> > > > -    mov     [rsp+4], cx
> > > > -    jmp     dword far [rsp]
> > > > +    push    rax
> > > > +    DB 0x48, 0xcb                      ; retfq
> > > >  setCodeSelectorLongJump:
> > > > -    add     rsp, 0x10
> > > >      ret
> > > >
> > > >  ;------------------------------------------------------------------------------
> > > > --
> > > > 2.16.2.windows.1
> > > >
> > > >
> > > >
> > > > 
> > > >