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Date: Mon, 8 Apr 2024 13:03:54 +0200
From: Andrew Jones <ajones@ventanamicro.com>
To: =?utf-8?B?Q2zDqW1lbnQgTMOpZ2Vy?= <cleger@rivosinc.com>
Cc: Deepak Gupta <debug@rivosinc.com>, Jonathan Corbet <corbet@lwn.net>, 
	Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt <palmer@dabbelt.com>, 
	Albert Ou <aou@eecs.berkeley.edu>, Conor Dooley <conor@kernel.org>, Rob Herring <robh@kernel.org>, 
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	kvm-riscv@lists.infradead.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH 0/5] Add parsing for Zimop ISA extension
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On Mon, Apr 08, 2024 at 10:01:12AM +0200, Clément Léger wrote:
> 
> 
> On 05/04/2024 19:33, Deepak Gupta wrote:
> > On Fri, Apr 5, 2024 at 8:26 AM Andrew Jones <ajones@ventanamicro.com> wrote:
> >>
> >> On Thu, Apr 04, 2024 at 12:32:46PM +0200, Clément Léger wrote:
> >>> The Zimop ISA extension was ratified recently. This series adds support
> >>> for parsing it from riscv,isa, hwprobe export and kvm support for
> >>> Guest/VM.
> >>
> >> I'm not sure we need this. Zimop by itself isn't useful, so I don't know
> >> if we need to advertise it at all. When an extension comes along that
> >> redefines some MOPs, then we'll advertise that extension, but the fact
> >> Zimop is used for that extension is really just an implementation detail.
> > 
> > Only situation I see this can be useful is this:--
> > 
> > An implementer, implemented Zimops in CPU solely for the purpose that they can
> > run mainline distro & packages on their hardware and don't want to leverage any
> > feature which are built on top of Zimop.
> 
> Yes, the rationale was that some binaries using extensions that overload
> MOPs could still be run. With Zimop exposed, the loader could determine
> if the binary can be executed without potentially crashing. We could
> also let the program run anyway but the execution could potentially
> crash unexpectedly, which IMHO is not really good for the user
> experience nor for debugging. I already think that the segfaults which
> happens when executing binaries that need some missing extension are not
> so easy to debug, so better add more guards.

OK. It's only one more extension out of dozens, so I won't complain more,
but I was thinking that binaries that use particular extensions would
check for those particular extensions (step 2), rather than Zimop.

Thanks,
drew

> 
> > 
> > As an example zicfilp and zicfiss are dependent on zimops. glibc can
> > do following
> > 
> > 1) check elf header if binary was compiled with zicfiss and zicfilp,
> > if yes goto step 2, else goto step 6.
> > 2) check if zicfiss/zicfilp is available in hw via hwprobe, if yes
> > goto step 5. else goto step 3
> > 3) check if zimop is available via hwprobe, if yes goto step 6, else goto step 4
> 
> I think you meant step 5 rather than step 6.
> 
> Clément
> 
> > 4) This binary won't be able to run successfully on this platform,
> > issue exit syscall. <-- termination
> > 5) issue prctl to enable shadow stack and landing pad for current task
> > <-- enable feature
> > 6) let the binary run <-- let the binary run because no harm can be done