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Date: Fri, 07 Jan 2022 23:28:44 +0530
From: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH 3/3] powerpc/bpf: Reallocate BPF registers to volatile
 registers when possible on PPC64
To: Christophe Leroy <christophe.leroy@csgroup.eu>, Jordan Niethe
 <jniethe5@gmail.com>, linuxppc-dev@lists.ozlabs.org
References: <20210727065539.299598-1-jniethe5@gmail.com>
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Christophe Leroy wrote:
>=20
>=20
> Le 27/07/2021 =C3=A0 08:55, Jordan Niethe a =C3=A9crit=C2=A0:
>> Implement commit 40272035e1d0 ("powerpc/bpf: Reallocate BPF registers to
>> volatile registers when possible on PPC32") for PPC64.
>>=20
>> When the BPF routine doesn't call any function, the non volatile
>> registers can be reallocated to volatile registers in order to avoid
>> having to save them/restore on the stack. To keep track of which
>> registers can be reallocated to make sure registers are set seen when
>> used.
>=20
> Maybe you could try and do as on PPC32, try to use r0 as much as possible=
 instead of TMP regs.
> r0 needs to be used carefully because for some instructions (ex: addi, lw=
z, etc) r0 means 0 instead=20
> of register 0, but it would help freeing one more register in several cas=
es.

Yes, but I think the utility of register re-mapping is debatable on=20
ppc64 since we are using NVRs only for BPF NVRs. Unlike the savings seen=20
with the test case shown in the commit description (and with other test=20
programs in test_bpf), most real world BPF programs will be generated by=20
llvm which will only use the NVRs if necessary. I also suspect that most=20
BPF programs will end up making at least one helper call.

On ppc32 though, there is value in re-mapping registers, especially=20
BPF_REG_AX and TMP_REG, and to a lesser extent, BPF_REG_5, since those=20
are volatile BPF registers and can be remapped regardless of a helper=20
call.


- Naveen