linux-kernel - Re: [PATCH] pvcalls-front: 64-bit align flags

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Message-ID: <alpine.DEB.2.10.1802271349280.4239@sstabellini-ThinkPad-X260>
Date:   Tue, 27 Feb 2018 13:50:27 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>, jgross@...e.com,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pvcalls-front: 64-bit align flags

On Tue, 27 Feb 2018, Boris Ostrovsky wrote:
> On 02/27/2018 04:32 PM, Stefano Stabellini wrote:
> > 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@...reto.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.
> 
> OK.
> 
> What about adding  __attribute__((aligned(8))) to both (with a comment
> explaining reasoning)?

That's fine by me


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