[PATCH v2] riscv, bpf: Optimize zextw insn with Zba extension

[Wang, Xiao W]

> -----Original Message-----
> From: Conor Dooley <conor.dooley at microchip.com>
> Sent: Wednesday, May 15, 2024 5:33 PM
> To: Andrew Jones <ajones at ventanamicro.com>
> Cc: Wang, Xiao W <xiao.w.wang at intel.com>; paul.walmsley at sifive.com;
> palmer at dabbelt.com; aou at eecs.berkeley.edu; luke.r.nels at gmail.com;
> xi.wang at gmail.com; bjorn at kernel.org; ast at kernel.org;
> daniel at iogearbox.net; andrii at kernel.org; martin.lau at linux.dev;
> eddyz87 at gmail.com; song at kernel.org; yonghong.song at linux.dev;
> john.fastabend at gmail.com; kpsingh at kernel.org; sdf at google.com;
> haoluo at google.com; jolsa at kernel.org; linux-riscv at lists.infradead.org; linux-
> kernel at vger.kernel.org; bpf at vger.kernel.org; pulehui at huawei.com; Li,
> Haicheng <haicheng.li at intel.com>; conor at kernel.org; Ben Dooks
> <ben.dooks at codethink.co.uk>
> Subject: Re: [PATCH v2] riscv, bpf: Optimize zextw insn with Zba extension
> 
> On Wed, May 15, 2024 at 09:19:46AM +0100, Conor Dooley wrote:
> > On Tue, May 14, 2024 at 03:37:02PM +0200, Andrew Jones wrote:
> > > On Tue, May 14, 2024 at 07:36:04AM GMT, Wang, Xiao W wrote:
> > > > > From: Andrew Jones <ajones at ventanamicro.com>
> > >> > > > +config RISCV_ISA_ZBA
> > > > > > +	bool "Zba extension support for bit manipulation instructions"
> > > > > > +	depends on TOOLCHAIN_HAS_ZBA
> > > > >
> > > > > We handcraft the instruction, so why do we need toolchain support?
> > > >
> > > > Good point, we don't need toolchain support for this bpf jit case.
> > > >
> > > > >
> > > > > > +	depends on RISCV_ALTERNATIVE
> > > > >
> > > > > Also, while riscv_has_extension_likely() will be accelerated with
> > > > > RISCV_ALTERNATIVE, it's not required.
> > > >
> > > > Agree, it's not required. For this bpf jit case, we should drop these two
> dependencies.
> > > >
> > > > BTW, Zbb is used in bpf jit, the usage there also doesn't depend on
> toolchain and
> > > > RISCV_ALTERNATIVE, but the Kconfig for RISCV_ISA_ZBB has forced the
> dependencies
> > > > due to Zbb assembly programming elsewhere.
> > > > Maybe we could just dynamically check the existence of RISCV_ISA_ZB*
> before jit code
> > > > emission? or introduce new config options for bpf jit? I prefer the first
> method and
> > > > welcome any comments.
> > >
> > > My preferences is to remove as much of the TOOLCHAIN_HAS_ stuff as
> > > possible. We should audit the extensions which have them to see if
> > > they're really necessary.
> >
> > While I think it is reasonable to allow the "RISCV_ISA_ZBB" option to
> > control whether or not bpf is allowed to use it for optimisations, only
> > allowing bpf to do that if there's toolchain support feels odd to me..
> > Maybe we need to sorta steal from Charlie's patchset and introduce
> > some hidden options that have the toolchain dep that are used by the
> > alternative macros etc?
> >
> > I'll have a poke at how bad that looks I think.
> 
> I don't love this, in particular my option naming, but it would allow
> the Zbb optimisations in the kernel to not depend on toolchain support
> while not muddying the Kconfig waters for users:
> https://git.kernel.org/pub/scm/linux/kernel/git/conor/linux.git/commit/?h=ri
> scv-zbb_split

In that patch, I think the bpt jit part should check IS_ENABLED(CONFIG_RISCV_ISA_ZBB)
rather than IS_ENABLED(CONFIG_RISCV_ISA_ZBB_ALT).

> A similar model could be followed if there were to be some
> optimisations for Zba in the future that do require toolchain support:

Though this model introduces extra hidden Kconfig option, it does provide finer 
config granularity. This should be a separate patch in the future, we can discuss about
the option naming there.

BRs,
Xiao