Date:   Thu, 26 Jan 2023 11:19:25 -0600
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     Andrew Donnellan <ajd@linux.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-integrity@vger.kernel.org,
        sudhakar@linux.ibm.com, bgray@linux.ibm.com, erichte@linux.ibm.com,
        gregkh@linuxfoundation.org, nayna@linux.ibm.com,
        linux-kernel@vger.kernel.org, zohar@linux.ibm.com,
        gjoyce@linux.ibm.com, joel@jms.id.au, ruscur@russell.cc,
        gcwilson@linux.ibm.com
Subject: Re: [PATCH v4 02/24] powerpc/pseries: Fix alignment of PLPKS structures and buffers
Message-ID: <20230126171925.GN25951@gate.crashing.org>
References: <20230120074306.1326298-1-ajd@linux.ibm.com> <20230120074306.1326298-3-ajd@linux.ibm.com> <87pmb2pxpa.fsf@mpe.ellerman.id.au>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87pmb2pxpa.fsf@mpe.ellerman.id.au>
User-Agent: Mutt/1.4.2.3i
Precedence: bulk

On Thu, Jan 26, 2023 at 12:09:53AM +1100, Michael Ellerman wrote:
> Andrew Donnellan <ajd@linux.ibm.com> writes:
> > A number of structures and buffers passed to PKS hcalls have alignment
> > requirements, which could on occasion cause problems:
> >
> > - Authorisation structures must be 16-byte aligned and must not cross a
> >   page boundary
> >
> > - Label structures must not cross page boundaries
> >
> > - Password output buffers must not cross page boundaries
> >
> > Round up the allocations of these structures/buffers to the next power of
> > 2 to make sure this happens.
> 
> It's not the *next* power of 2, it's the *nearest* power of 2, including
> the initial value if it's already a power of 2.

It's not the nearest either, the nearest power of two to 65 is 64.  You
could say "but, round up" to which I would say "round?"  :-P

"Adjust the allocation size to be the smallest power of two greater than
or equal to the given size."

"Pad to a power of two" in shorthand.  "Padded to a power of two if
necessary" if you want to emphasise it can be a no-op.


Segher