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From: "Erdem Aktas" <erdemaktas@google.com>
Date: Tue, 14 Sep 2021 05:16:48 +0300
Message-ID: <CAAYXXYyyVF4nRQM6_cNuY5MRQZUss38oBoQE-1CVcPFwmyA-xA@mail.gmail.com>
Subject: Re: [PATCH 03/23] OvmfPkg/ResetVector: Enable Intel TDX in ResetVector of Ovmf
To: "Xu, Min M" <min.m.xu@intel.com>
Cc: edk2-devel-groups-io <devel@edk2.groups.io>, Ard Biesheuvel <ardb+tianocore@kernel.org>, 
	"Justen, Jordan L" <jordan.l.justen@intel.com>, Brijesh Singh <brijesh.singh@amd.com>, 
	James Bottomley <jejb@linux.ibm.com>, "Yao, Jiewen" <jiewen.yao@intel.com>, 
	Tom Lendacky <thomas.lendacky@amd.com>
Content-Type: text/plain; charset="UTF-8"

On Mon, Sep 13, 2021 at 9:06 AM Xu, Min M <min.m.xu@intel.com> wrote:
>
> > > +TdxApWait:
> > > +    cmp     byte[TDX_WORK_AREA_PGTBL_READY], 0
> > > +    je      TdxApWait
> > Don't we need memory fence before  je      TdxApWait. I did not check
> > what the compiler generates for this loop.
> Below is the code compiler generated for this loop. (VS2017/release)
>    106                                                          <1> TdxApWait:
>    107 0000070B 803D04B0800000      <1>     cmp     byte[TDX_WORK_AREA_PGTBL_READY], 0
>    108 00000712 74F7                             <1>     je          TdxApWait
>    109 00000714 EB17                             <1>     jmp      ExitInitTdxWorkarea
>
> This is the code lfence is added.
>    106                                                          <1> TdxApWait:
>    107 0000070B 803D04B0800000      <1>     cmp     byte[TDX_WORK_AREA_PGTBL_READY], 0
>    108 00000712 0FAEE8                         <1>     lfence
>    109 00000715 74F4                              <1>     je         TdxApWait
>    110 00000717 EB17                              <1>     jmp     ExitInitTdxWorkarea
>
> I am not sure if lfence is needed.

Thanks Min! You are right, I also checked it with GCC5 and saw the same output.
Thanks for checking it.

-Erdem