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

[Mark Rutland <mark.rutland@xxxxxxx>]

On Wed, Nov 27, 2019 at 10:31:54AM -0500, Pavel Tatashin wrote:
> 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:

Sorry, I meant that IRQs are disabled here.

The C wrapper calls __uaccess_ttbr0_enable(), which calls
local_irq_save() and local_irq_restore(). Those are pointless in the
bowels of the entry code, and potentially expensive if IRQ prio masking
is in use.

I'd rather not add more out-of-line C code calls here right now as I'd
prefer to factor out the logic to C in a better way.

> > 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.

That sounds fine to me,

Thanks,
Mark.

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