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    SubjectRe: [PATCH v3 1/2] RISC-V: Define sys_riscv_flush_icache when SMP=n
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    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.

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      Last update: 2018-08-10 22:56    [W:4.835 / U:0.008 seconds]
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