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SubjectRe: linux-next: manual merge of the tip tree with Linus' tree
On Tue, Feb 06, 2018 at 12:52:34PM +0000, Mathieu Desnoyers wrote:
> ----- On Feb 5, 2018, at 7:40 PM, Stephen Rothwell sfr@canb.auug.org.au wrote:
>
> > Hi all,
> >
> > Today's linux-next merge of the tip tree got a conflict in:
> >
> > arch/arm64/kernel/entry.S
> >
> > between commit:
> >
> > 4bf3286d29f3 ("arm64: entry: Hook up entry trampoline to exception vectors")
> >
> > from Linus' tree and commit:
> >
> > f1e3a12b6543 ("membarrier/arm64: Provide core serializing command")
> >
> > from the tip tree.
> >
> > I fixed it up (probably not completely - see below) and can carry the
> > fix as necessary. This is now fixed as far as linux-next is concerned,
> > but any non trivial conflicts should be mentioned to your upstream
> > maintainer when your tree is submitted for merging. You may also want
> > to consider cooperating with the maintainer of the conflicting tree to
> > minimise any particularly complex conflicts.
>
> The change introduced by 4bf3286d29 "arm64: entry: Hook up entry trampoline
> to exception vectors" adds a trampoline as mechanism for return:
>
> - eret // return to kernel
> +
> +#ifndef CONFIG_UNMAP_KERNEL_AT_EL0
> + eret
> +#else
> + .if \el == 0
> + bne 4f
> + msr far_el1, x30
> + tramp_alias x30, tramp_exit_native
> + br x30
> +4:
> + tramp_alias x30, tramp_exit_compat
> + br x30
> + .else
> + eret
> + .endif
> +#endif
>
> Which means that with CONFIG_UNMAP_KERNEL_AT_EL0, for return to EL0,
> the eret is in the trampoline:
>
> .macro tramp_exit, regsize = 64
> adr x30, tramp_vectors
> msr vbar_el1, x30
> tramp_unmap_kernel x30
> .if \regsize == 64
> mrs x30, far_el1
> .endif
> eret
> .endm
>
> ENTRY(tramp_exit_native)
> tramp_exit
> END(tramp_exit_native)
>
> ENTRY(tramp_exit_compat)
> tramp_exit 32
> END(tramp_exit_compat)
>
>
> One approach I would consider for this is to duplicate this
> comment and add it just above the "eret" instruction within the
> macro:
>
> /*
> * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on eret context synchronization
> * when returning from IPI handler, and when returning to user-space.
> */
>
> Or perhaps Will has something else in mind ?

To be honest with you, I'd just drop the comment entirely. entry.S is
terrifying these days and nobody should have to go in there to understand
why we select ARCH_HAS_MEMBARRIER_SYNC_CORE. If you really feel a justification
is needed, I'd be happy with a line in the Kconfig file.

Will

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