Re: [Xen-devel] [PATCH v2 3/3] arm64: remove the rest of asm-uaccess.h

[Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>]

On Wed, Nov 27, 2019 at 10:12 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Thu, Nov 21, 2019 at 09:24:06PM -0500, Pavel Tatashin wrote:
> > The __uaccess_ttbr0_disable and __uaccess_ttbr0_enable,
> > are the last two macros defined in asm-uaccess.h.
> >
> > Replace them with C wrappers and call C functions from
> > kernel_entry and kernel_exit.
>
> For now, please leave those as-is.
>
> I don't think we want to have out-of-line C wrappers in the middle of
> the entry assembly where we don't have a complete kernel environment.
> The use in entry code can also assume non-preemptibility, while the C
> functions have to explcitily disable that.

I do not understand, if C function is called form non-preemptible
context it stays non-preemptible. kernel_exit already may call C
functions around the time __uaccess_ttbr0_enable is called (it may
call post_ttbr_update_workaround), and that C functions does not do
explicit preempt disable:

> We can certainly remove the includes of <asm/asm-uaccess.h> elsewhere,
> and maybe fold the macros into entry.S if it's not too crowded.

I can do this as a separate patch.

Thank you,
Pasha

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