linux-kernel - Re: [External] [PATCH RFC/RFT v2 3/4] riscv: Stop emitting preventive sfence.vma for new vmalloc mappings

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Date: Tue, 4 Jun 2024 13:44:15 +0200
From: Alexandre Ghiti <alexghiti@...osinc.com>
To: Conor Dooley <conor@...nel.org>
Cc: yunhui cui <cuiyunhui@...edance.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Thomas Bogendoerfer <tsbogend@...ha.franken.de>, 
	Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Andrew Morton <akpm@...ux-foundation.org>, Ved Shanbhogue <ved@...osinc.com>, 
	Matt Evans <mev@...osinc.com>, Dylan Jhong <dylan@...estech.com>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, 
	linux-riscv@...ts.infradead.org, linux-mm@...ck.org
Subject: Re: [External] [PATCH RFC/RFT v2 3/4] riscv: Stop emitting preventive
 sfence.vma for new vmalloc mappings

On Tue, Jun 4, 2024 at 10:52 AM Conor Dooley <conor@...nel.org> wrote:
>
> On Tue, Jun 04, 2024 at 09:17:26AM +0200, Alexandre Ghiti wrote:
> > On Tue, Jun 4, 2024 at 9:15 AM Alexandre Ghiti <alexghiti@...osinc.com> wrote:
> > > On Tue, Jun 4, 2024 at 8:21 AM yunhui cui <cuiyunhui@...edance.com> wrote:
> > > >
> > > > As for the current status of the patch, there are two points that can
> > > > be optimized:
> > > > 1. Some chip hardware implementations may not cache TLB invalid
> > > > entries, so it doesn't matter whether svvptc is available or not. Can
> > > > we consider adding a CONFIG_RISCV_SVVPTC to control it?
> >
> > That would produce a non-portable kernel. But I'm not opposed to that
> > at all, let me check how we handle other extensions. Maybe @Conor
> > Dooley has some feedback here?
>
> To be honest, not really sure what to give feedback on. Could you
> elaborate on exactly what the option is going to do? Given the
> portability concern, I guess you were proposing that the option would
> remove the preventative fences, rather than your current patch that
> removes them via an alternative?

No no, I won't do that, we need a generic kernel for distros so that's
not even a question. What Yunhui was asking about (to me) is: can we
introduce a Kconfig option to always remove the preventive fences,
bypassing the use of alternatives altogether?

To me, it won't make a difference in terms of performance. But if we
already offer such a possibility for other extensions, well I'll do
it. Otherwise, the question is: should we start doing that?

> I don't think we have any extension
> related options that work like that at the moment, and making that an
> option will just mean that distros that look to cater for multiple
> platforms won't be able to turn it on.
>
> Thanks,
> Conor.