linux-kernel - Re: [PATCH v4 02/24] powerpc/pseries: Fix alignment of PLPKS structures and buffers

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Date:   Thu, 26 Jan 2023 11:19:25 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Andrew Donnellan <ajd@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-integrity@...r.kernel.org,
        sudhakar@...ux.ibm.com, bgray@...ux.ibm.com, erichte@...ux.ibm.com,
        gregkh@...uxfoundation.org, nayna@...ux.ibm.com,
        linux-kernel@...r.kernel.org, zohar@...ux.ibm.com,
        gjoyce@...ux.ibm.com, joel@....id.au, ruscur@...sell.cc,
        gcwilson@...ux.ibm.com
Subject: Re: [PATCH v4 02/24] powerpc/pseries: Fix alignment of PLPKS structures and buffers

On Thu, Jan 26, 2023 at 12:09:53AM +1100, Michael Ellerman wrote:
> Andrew Donnellan <ajd@...ux.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