Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BB9562168D
Date:   Tue, 27 Feb 2018 13:32:32 -0800 (PST)
From:   Stefano Stabellini <sstabellini@kernel.org>
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>
cc:     Stefano Stabellini <sstabellini@kernel.org>, jgross@suse.com,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] pvcalls-front: 64-bit align flags
In-Reply-To: <b7ec592a-bbd1-48d3-1a1f-92a8ed4a1e48@oracle.com>
Message-ID: <alpine.DEB.2.10.1802271328140.4239@sstabellini-ThinkPad-X260>
References: <alpine.DEB.2.10.1802271150020.4239@sstabellini-ThinkPad-X260> <b7ec592a-bbd1-48d3-1a1f-92a8ed4a1e48@oracle.com>
User-Agent: Alpine 2.10 (DEB 1266 2009-07-14)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, 27 Feb 2018, Boris Ostrovsky wrote:
> On 02/27/2018 02:54 PM, Stefano Stabellini wrote:
> > We are using test_and_* operations on the status and flag fields of
> > struct sock_mapping. However, these functions require the operand to be
> > 64-bit aligned on arm64. Currently, only status is 64-bit aligned.
> >
> > Make flags 64-bit aligned by introducing an explicit padding field.
> >
> > Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
> >
> > diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> > index ca5b773..aa07b2a 100644
> > --- a/drivers/xen/pvcalls-front.c
> > +++ b/drivers/xen/pvcalls-front.c
> > @@ -78,6 +78,7 @@ struct sock_mapping {
> >  #define PVCALLS_STATUS_BIND          1
> >  #define PVCALLS_STATUS_LISTEN        2
> >  			uint8_t status;
> > +			uint8_t pad[7];
> 
> Does this guarantee alignment (for either status or flag)?

Yes: status is part of a struct and a union. Unions and structs have the
alignment of their most aligned type. In this case they are 64-bit
aligned, as some of the fields are pointers.

The padding makes sure that flags is 1+7 bytes from it.

 
> >  		/*
> >  		 * Internal state-machine flags.
> >  		 * Only one accept operation can be inflight for a socket.
>