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

[Conor Dooley]

On Tue, Jun 04, 2024 at 01:44:15PM +0200, Alexandre Ghiti wrote:
> On Tue, Jun 4, 2024 at 10:52 AM Conor Dooley <conor@xxxxxxxxxx> 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@xxxxxxxxxxxx> wrote:
> > > > On Tue, Jun 4, 2024 at 8:21 AM yunhui cui <cuiyunhui@xxxxxxxxxxxxx> 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?

We don't do that for other extensions yet, because currently all the
extensions we have options for are additive. There's like 3 alternative
patchsites, and they are all just one nop? I don't see the point of
having a Kconfig knob for that.

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