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From: "Dhaval Sharma" <dhaval@rivosinc.com>
Date: Wed, 1 Nov 2023 22:35:29 +0530
Message-ID: <CAAxYnhRbFDD3QaMba7DgkhzmKaKwQ8A=pFe7MiKDx5TmytLbbg@mail.gmail.com>
Subject: Re: [edk2-devel] [PATCH v7 5/5] OvmfPkg/RiscVVirt: Override for RV CPU Features
To: "Warkentin, Andrei" <andrei.warkentin@intel.com>, devel@edk2.groups.io
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Hi Andrei,
Are you suggesting:
1. We have a generic PCD to store the address of RVFeatures bitfield.
2. It gets populated at some point during initialization with let's say
some kind of global variable address which keeps this bitfield.
3. SEC/DXE phases deref this address and use where needed?
Is there a reference I can take a look at?
Assuming my above understanding is correct, the whole idea was to keep
implementation based on simple PCD such that it can be used easily as an
override mechanism. The reason to keep it "enabled" by default in MDE is
that if m-mode decides to keep it disabled it is anyways going to remain
that way. So practically this PCD is going to be useful only in cases where
the user wants to "Override Disable". LMK if you still think we should
modify the implementation.
=3DD
On Tue, Oct 31, 2023 at 10:31=E2=80=AFPM Warkentin, Andrei <
andrei.warkentin@intel.com> wrote:
> I think I misunderstood the intent. Reviewing the full patchset, it seems
> this is necessary to avoid using the new CMO path in the Virt platform
> (since the default value is all FFs). Shouldn=E2=80=99t the default Pcd v=
alue here
> be all 0=E2=80=99s =E2=80=93 i.e. CMO or any other feature use becomes =
=E2=80=9Copt in=E2=80=9D instead of
> =E2=80=9Copt out=E2=80=9D?
>
>
>
> It also seems that encoding the meaning inside the bit positions is a bit=
=E2=80=A6
> obscure. Have you considered storing a pointer to a struct with bitfields
> instead? You could then change the logic to be something like =E2=80=9CIf
> PcdPtrValue !=3D NULL && ((struct cast *) PcdPtrValue)->LegibleFieldName=
=E2=80=9D. I
> think this would do wonders for code maintainability. The cost of course =
is
> in having to initialize the Pcd now at runtime, and the additional
> dereference, but that seems like a low cost all things considered.
>
>
>
> *From:* Dhaval Sharma <dhaval@rivosinc.com>
> *Sent:* Tuesday, October 31, 2023 1:13 AM
> *To:* Warkentin, Andrei <andrei.warkentin@intel.com>; devel@edk2.groups.i=
o
> *Subject:* Re: [edk2-devel] [PATCH v7 5/5] OvmfPkg/RiscVVirt: Override
> for RV CPU Features
>
>
>
> Thanks. This PCD is for Virt platform only. Or maybe I am missing the
> point.
>
--=20
Thanks!
=3DD
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<div dir=3D"ltr">Hi Andrei,<div>Are you suggesting:</div><div><ol><li>We ha=
ve a generic PCD to store the address of RVFeatures bitfield.</li><li>It ge=
ts populated at some point during initialization with let's say some ki=
nd of global variable address which keeps this bitfield.</li><li>SEC/DXE ph=
ases deref this address and use where needed?</li></ol><div>Is there a refe=
rence I can take a look at?</div></div><div><br></div><div>Assuming my abov=
e understanding is correct, the whole idea was to keep implementation based=
on simple PCD such that it can be used easily as an override mechanism. Th=
e reason to keep it "enabled" by default in MDE is that if m-mode=
decides to keep it disabled it is anyways going to remain that way. So pra=
ctically this PCD is going to be useful only in cases where the user wants =
to "Override Disable". LMK if you still think we should modify th=
e implementation.</div><div><br></div><div>=3DD</div></div><br><div class=
=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Tue, Oct 31, 2023=
at 10:31=E2=80=AFPM Warkentin, Andrei <<a href=3D"mailto:andrei.warkent=
in@intel.com">andrei.warkentin@intel.com</a>> wrote:<br></div><blockquot=
e class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px s=
olid rgb(204,204,204);padding-left:1ex"><div class=3D"msg-38786047620848271=
67">
<div lang=3D"EN-US" style=3D"overflow-wrap: break-word;">
<div class=3D"m_-3878604762084827167WordSection1">
<p class=3D"MsoNormal">I think I misunderstood the intent. Reviewing the fu=
ll patchset, it seems this is necessary to avoid using the new CMO path in =
the Virt platform (since the default value is all FFs). Shouldn=E2=80=99t t=
he default Pcd value here be all 0=E2=80=99s =E2=80=93 i.e.
CMO or any other feature use becomes =E2=80=9Copt in=E2=80=9D instead of =
=E2=80=9Copt out=E2=80=9D?<u></u><u></u></p>
<p class=3D"MsoNormal"><u></u>=C2=A0<u></u></p>
<p class=3D"MsoNormal">It also seems that encoding the meaning inside the b=
it positions is a bit=E2=80=A6 obscure. Have you considered storing a point=
er to a struct with bitfields instead? You could then change the logic to b=
e something like =E2=80=9CIf PcdPtrValue !=3D NULL &&
((struct cast *) PcdPtrValue)->LegibleFieldName=E2=80=9D. I think this =
would do wonders for code maintainability. The cost of course is in having =
to initialize the Pcd now at runtime, and the additional dereference, but t=
hat seems like a low cost all things considered.<u></u><u></u></p>
<p class=3D"MsoNormal"><b><span><u></u>=C2=A0<u></u></span></b></p>
<p class=3D"MsoNormal"><b><span>From:</span></b><span> Dhaval Sharma <<a=
href=3D"mailto:dhaval@rivosinc.com" target=3D"_blank">dhaval@rivosinc.com<=
/a>>
<br>
<b>Sent:</b> Tuesday, October 31, 2023 1:13 AM<br>
<b>To:</b> Warkentin, Andrei <<a href=3D"mailto:andrei.warkentin@intel.c=
om" target=3D"_blank">andrei.warkentin@intel.com</a>>; <a href=3D"mailto=
:devel@edk2.groups.io" target=3D"_blank">devel@edk2.groups.io</a><br>
<b>Subject:</b> Re: [edk2-devel] [PATCH v7 5/5] OvmfPkg/RiscVVirt: Override=
for RV CPU Features</span><u></u><u></u></p>
<div style=3D"border-top:none;border-right:none;border-bottom:none;border-l=
eft:1.5pt solid blue;padding:0in 0in 0in 4pt">
<p class=3D"MsoNormal"><u></u>=C2=A0<u></u></p>
<p class=3D"MsoNormal"><span>Thanks. This PCD is for Virt platform only. Or=
maybe I am missing the point.
<u></u><u></u></span></p>
</div>
</div>
</div>
</div></blockquote></div><br clear=3D"all"><div><br></div><span class=3D"gm=
ail_signature_prefix">-- </span><br><div dir=3D"ltr" class=3D"gmail_signatu=
re"><div dir=3D"ltr">Thanks!<div>=3DD</div></div></div>
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