Re: [Xen-devel] [RFC PATCH v3 04/10] arm/mem_access: Add short-descriptor pte typedefs

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andrew,

On 06/15/2017 01:03 PM, Andrew Cooper wrote:
> On 15/06/17 12:05, Sergej Proskurin wrote:
> > The current implementation does not provide appropriate types for
> > short-descriptor translation table entries. As such, this commit adds new
> > types, which simplify managing the respective translation table entries.
> >
> > Signed-off-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
> > ---
> > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > Cc: Julien Grall <julien.grall@xxxxxxx>
> > ---
> > v3: Add more short-descriptor related pte typedefs that will be used by
> >      the following commits.
> > ---
> >   xen/include/asm-arm/page.h | 104 +++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 104 insertions(+)
> >
> > diff --git a/xen/include/asm-arm/page.h b/xen/include/asm-arm/page.h
> > index e2e4b597a5..7a4aa64144 100644
> > --- a/xen/include/asm-arm/page.h
> > +++ b/xen/include/asm-arm/page.h
> > @@ -205,6 +205,110 @@ typedef union {
> >       lpae_walk_t walk;
> >   } lpae_t;
> >   
> > +/*
> > + *  Comprises bits of the level 1 short-descriptor format representing
> > + *  a section.
> > + */
> > +typedef struct __packed {
> > +    unsigned int pxn:1;         /* Privileged Execute Never */
> (I'm not an ARM maintainer, but) can I recommend using bool bitfields
> for boolean fields like this.
I was not aware it was possible to do boolean fields. I am all for it.

> There's no difference when using the structure for reading data, but it
> helps reduce subtle bugs such as
>
> foo.pxn = (flags & NO_EXECUTE);
>
> when using the structures to create a new value, or modify existing ones.
>
> ~Andrew
Cheers,

--
Julien Grall

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