linux-kernel - Re: [PATCH] powerpc/bpf: Always reallocate BPF_REG_5, BPF_REG_AX and TMP

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Message-Id: <1642156339.pkhk6znoh0.naveen@linux.ibm.com>
Date:   Fri, 14 Jan 2022 16:07:11 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul
 Mackerras <paulus@...ba.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] powerpc/bpf: Always reallocate BPF_REG_5, BPF_REG_AX and
 TMP_REG when possible

Christophe Leroy wrote:
> 
> 
> Le 14/01/2022 à 08:58, Naveen N. Rao a écrit :
>> Christophe Leroy wrote:
>>> BPF_REG_5, BPF_REG_AX and TMP_REG are mapped on non volatile registers
>>> because there are not enough volatile registers, but they don't need
>>> to be preserved on function calls.
>>>
>>> So when some volatile registers become available, those registers can
>>> always be reallocated regardless of whether SEEN_FUNC is set or not.
>>>
>>> Suggested-by: Naveen N. Rao <naveen.n.rao@...ux.ibm.com>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>>> ---
>>>  arch/powerpc/net/bpf_jit.h        |  3 ---
>>>  arch/powerpc/net/bpf_jit_comp32.c | 14 +++++++++++---
>>>  2 files changed, 11 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
>>> index b20a2a83a6e7..b75507fc8f6b 100644
>>> --- a/arch/powerpc/net/bpf_jit.h
>>> +++ b/arch/powerpc/net/bpf_jit.h
>>> @@ -127,9 +127,6 @@
>>>  #define SEEN_FUNC    0x20000000 /* might call external helpers */
>>>  #define SEEN_TAILCALL    0x40000000 /* uses tail calls */
>>>
>>> -#define SEEN_VREG_MASK    0x1ff80000 /* Volatile registers r3-r12 */
>>> -#define SEEN_NVREG_MASK    0x0003ffff /* Non volatile registers 
>>> r14-r31 */
>>> -
>>>  #ifdef CONFIG_PPC64
>>>  extern const int b2p[MAX_BPF_JIT_REG + 2];
>>>  #else
>>> diff --git a/arch/powerpc/net/bpf_jit_comp32.c 
>>> b/arch/powerpc/net/bpf_jit_comp32.c
>>> index d3a52cd42f53..cfec42c8a511 100644
>>> --- a/arch/powerpc/net/bpf_jit_comp32.c
>>> +++ b/arch/powerpc/net/bpf_jit_comp32.c
>>> @@ -77,14 +77,22 @@ static int bpf_jit_stack_offsetof(struct 
>>> codegen_context *ctx, int reg)
>>>      return BPF_PPC_STACKFRAME(ctx) - 4;
>>>  }
>>>
>>> +#define SEEN_VREG_MASK        0x1ff80000 /* Volatile registers r3-r12 */
>>> +#define SEEN_NVREG_FULL_MASK    0x0003ffff /* Non volatile registers 
>>> r14-r31 */
>>> +#define SEEN_NVREG_TEMP_MASK    0x00001e01 /* BPF_REG_5, BPF_REG_AX, 
>>> TMP_REG */
>> 
>> Could have been named better: SEEN_NVREG_BPF_VGER_MASK, or such.
> 
> Yes, I was suffering from a lack of inspiration.
> 
> What does BPF_VGER mean ?

That I was suffering from a lack of caffeine.

I meant to suggest BPF_VREG, to indicate those are BPF volatile 
registers.


- Naveen