Re: [Xen-devel] [PATCH 1/3] arm/arm64/xen: use C inlines for privcmd_call

[Russell King - ARM Linux admin <linux@xxxxxxxxxxxxxxx>]

On Thu, Nov 21, 2019 at 07:39:22PM -0500, Pavel Tatashin wrote:
> > > That may be, but be very careful that you only use them in ARMv7-only
> > > code.  Using them elsewhere is unsafe as the domain register is used
> > > for other purposes, and merely blatting over it (as your
> > > uaccess_enable and uaccess_disable functions do) is unsafe.
> >
> > In fact, I'll turn that into a bit more than a suggestion.  I'll make
> > it a NAK on adding them to 32-bit ARM.
> >
> 
> That's fine, and I also did not want to change ARM 32-bit. But, do you
> have a suggestion how differentiate between arm64 and arm in
> include/xen/arm/hypercall.h without ugly ifdefs?

Sorry, I don't.

I'm surprised ARM64 doesn't have anything like that, but I suspect
that's because they don't need to do a save/restore type operation.
Whereas, 32-bit ARM does very much need the save/restore behaviour
(although not in this path.)

The problem is, turning uaccess_enable/disable into C code means
that it's open to being used elsewhere in the kernel (ooh, a couple
of useful looking functions that work on both architectures!  I can
use that too!) and then we end up with stuff breaking subtly.  It's
the potential for subtle breakage that is making me NAK the idea of
adding the inline C functions.

Given the two have diverged, the only answer is ifdefs, sorry.

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