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Subject: Re: [PATCH v3 06/35] OvmfPkg/XenResetVector: Add new entry point for Xen PVH
To: Anthony PERARD <anthony.perard@citrix.com>,
 Andrew Cooper <andrew.cooper3@citrix.com>
Cc: devel@edk2.groups.io, xen-devel@lists.xenproject.org,
 Ard Biesheuvel <ard.biesheuvel@linaro.org>,
 Jordan Justen <jordan.l.justen@intel.com>,
 Julien Grall <julien.grall@arm.com>, =?UTF-8?Q?Roger_Pau_Monn=c3=a9?=
 <roger.pau@citrix.com>
References: <20190704144233.27968-1-anthony.perard@citrix.com>
 <20190704144233.27968-7-anthony.perard@citrix.com>
 <a91c8d00-a40b-3906-6432-b9e0c2953843@citrix.com>
 <20190719102008.GB1208@perard.uk.xensource.com>
From: "Laszlo Ersek" <lersek@redhat.com>
Message-ID: <03cc01a5-d6a8-84b7-d382-d83fadea486b@redhat.com>
Date: Fri, 19 Jul 2019 16:33:31 +0200
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On 07/19/19 12:20, Anthony PERARD wrote:
> On Fri, Jul 05, 2019 at 02:57:06PM +0100, Andrew Cooper wrote:
>> On 04/07/2019 15:42, Anthony PERARD wrote:
>>> diff --git a/OvmfPkg/XenResetVector/Ia16/ResetVectorVtf0.asm b/OvmfPkg/XenResetVector/Ia16/ResetVectorVtf0.asm
>>> new file mode 100644
>>> index 0000000000..958195bc5e
>>> --- /dev/null
>>> +++ b/OvmfPkg/XenResetVector/Ia16/ResetVectorVtf0.asm
>>> +vtfSignature:
>>> +    DB      'V', 'T', 'F', 0
>>> +
>>> +ALIGN   16
>>> +
>>> +resetVector:
>>> +;
>>> +; Reset Vector
>>> +;
>>> +; This is where the processor will begin execution
>>> +;
>>> +    nop
>>> +    nop
>>
>> Why two nops?
> 
> I don't know, this is existing code that I duplicated to allow adding a
> new entry point. (I wanted to use --find-copies-harder when sending the
> patch, but forgot this time.

Not a big problem; while reviewing v3, I did such comparisons myself, in
my local clone. Feel free to skip "--find-copies-harder" when posting v4
too; I think there isn't much churn going on in parallel right now.

However, a new request for v4: please make sure that the new modules /
paths introduced by this patch set are covered in Maintainers.txt.

> This part of the chunk would not be there.)

Regarding the NOPs: all I can tell you is that they originate from
commit 8332983e2e33 ("UefiCpuPkg: Replace the un-necessary WBINVD
instruction at the reset vector with two NOPs in VTF0.", 2011-08-04).

Whether that change made sense back then, let alone if it makes sense
now: no clue.

Thanks
Laszlo