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

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Date:   Tue, 27 Feb 2018 16:45:08 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     jgross@...e.com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pvcalls-front: 64-bit align flags

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)?

-boris


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