From: Palmer Dabbelt <palmer@sifive.com>
To: linux@roeck-us.net
Cc: Christoph Hellwig <hch@infradead.org>,
aou@eecs.berkeley.edu, Andrew Waterman <andrew@sifive.com>,
Arnd Bergmann <arnd@arndb.de>,
linux-kernel@vger.kernel.org, linux@dominikbrodowski.net,
dan.carpenter@oracle.com, tklauser@distanz.ch,
linux-riscv@lists.infradead.org
Subject: Re: [PATCH v3 1/2] RISC-V: Define sys_riscv_flush_icache when SMP=n
Date: Fri, 10 Aug 2018 13:55:15 -0700 (PDT) [thread overview]
Message-ID: <mhng-f3c54437-a74c-4a33-b59e-84eaf3453c85@palmer-si-x1c4> (raw)
In-Reply-To: <20180810184715.GA11259@roeck-us.net>
On Fri, 10 Aug 2018 11:47:15 PDT (-0700), linux@roeck-us.net wrote:
> 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.
Well, the officially sanctioned way to obtain this functionality is via a vDSO
call. On non-SMP systems it will never make the system call. As a result we
thought we'd keep it out of the ABI, but after looking again it seems yucky to
do so. Here's the vDSO entry, for reference:
ENTRY(__vdso_flush_icache)
.cfi_startproc
#ifdef CONFIG_SMP
li a7, __NR_riscv_flush_icache
ecall
#else
fence.i
li a0, 0
#endif
ret
.cfi_endproc
ENDPROC(__vdso_flush_icache)
Note that glibc has a fallback to make the system call if it can't find the
vDSO entry, but then doesn't have a secondary fallback to emit a local fence.i
if the system call doesn't exist. It seems easier to fix the kernel to always
provide the syscall and just call it a bug.
next prev parent reply other threads:[~2018-08-10 20:55 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-09 22:19 RISC-V: Don't use a global include guard for uapi/asm/syscalls Palmer Dabbelt
2018-08-09 22:19 ` [PATCH v3 1/2] RISC-V: Define sys_riscv_flush_icache when SMP=n Palmer Dabbelt
2018-08-10 8:38 ` Christoph Hellwig
2018-08-10 18:27 ` Palmer Dabbelt
2018-08-10 18:47 ` Guenter Roeck
2018-08-10 20:55 ` Palmer Dabbelt [this message]
2018-08-14 13:35 ` Christoph Hellwig
2018-08-10 14:07 ` Guenter Roeck
2018-08-14 13:39 ` Christoph Hellwig
2018-08-20 23:30 ` Palmer Dabbelt
2018-08-09 22:19 ` [PATCH v3 2/2] RISC-V: Don't use a global include guard for uapi/asm/syscalls.h Palmer Dabbelt
2018-08-10 14:07 ` Guenter Roeck
2018-08-14 13:40 ` Christoph Hellwig
2018-08-20 23:30 ` Palmer Dabbelt
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