linux-kernel - Re: [PATCH v3 1/2] RISC-V: Define sys_riscv_flush

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Message-ID: <20180810184715.GA11259@roeck-us.net>
Date:   Fri, 10 Aug 2018 11:47:15 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Palmer Dabbelt <palmer@...ive.com>
Cc:     Christoph Hellwig <hch@...radead.org>, aou@...s.berkeley.edu,
        Andrew Waterman <andrew@...ive.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        linux@...inikbrodowski.net, dan.carpenter@...cle.com,
        tklauser@...tanz.ch, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v3 1/2] RISC-V: Define sys_riscv_flush_icache when SMP=n

On Fri, Aug 10, 2018 at 11:27:37AM -0700, Palmer Dabbelt wrote:
> On Fri, 10 Aug 2018 01:38:04 PDT (-0700), Christoph Hellwig wrote:
> >On Thu, Aug 09, 2018 at 03:19:51PM -0700, Palmer Dabbelt wrote:
> >>This would be necessary to make non-SMP builds work, but there is
> >>another error in the implementation of our syscall linkage that actually
> >>just causes sys_riscv_flush_icache to never build.  I've build tested
> >>this on allnoconfig and allnoconfig+SMP=y, as well as defconfig like
> >>normal.
> >
> >Would't it make sense to use COND_SYSCALL to stub out the syscall
> >for !SMP builds?
> 
> I'm not sure.  We can implement the syscall fine in !SMP, it's just that the
> vDSO is expected to always eat these calls because in non-SMP mode you can
> do a global fence.i by just doing a local fence.i (there's only one hart).
> 
> The original rationale behind not having the syscall in non-SMP mode was to
> limit the user ABI, but on looking again that seems like it's just a bit of
> extra complexity that doesn't help anything.  It's already been demonstrated

Doesn't this mean that some userspace code will only run if the kernel was
compiled for SMP ? I always thought that was unacceptable.

Guenter

> that nothing is checking the error because it's been silently slipping past
> userspace for six months, so the extra complexity seems like it'll just
> cause someone else to have to chase the bug in the future.
> 
> But I'm really OK either way.  Is there a precedent for what to do here?