Re: [Xen-devel] [PATCH v6 01/12] xen: allow only sizeof(bool) variables for boolean_param()

From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>
Cc: "Juergen Gross" <jgross@suse.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Wei Liu" <wl@xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Ian Jackson" <ian.jackson@eu.citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	xen-devel@lists.xenproject.org,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v6 01/12] xen: allow only sizeof(bool) variables for boolean_param()
Date: Mon, 9 Mar 2020 12:55:13 +0100	[thread overview]
Message-ID: <471c2f88-2ac2-823d-546f-6fc817e34ec8@suse.com> (raw)
In-Reply-To: <d3b83513-8fc2-634c-fa89-00df84c5c4fb@xen.org>

On 09.03.2020 12:43, Julien Grall wrote:
> On 26/02/2020 12:46, Juergen Gross wrote:
>> @@ -46,7 +48,8 @@ extern const struct kernel_param __param_start[], __param_end[];
>>       __kparam __setup_##_var = \
>>           { .name = __setup_str_##_var, \
>>             .type = OPT_BOOL, \
>> -          .len = sizeof(_var), \
>> +          .len = sizeof(_var) + \
>> +                 BUILD_BUG_ON_ZERO(sizeof(_var) != sizeof(bool)), \
> 
>  From my understanding, sizeof(bool) is not necessarily 1 (it can be 
> greater). While this is fine to use it in Xen, I think we want it to 
> always be one when exposed in the hypfs.

I don't think so: We want variable of type 'bool' to be updated
consistently (i.e. by a write to the full variable). Hence I
think sizeof(bool) is correct here. I can see though that the
hypercall interface then gains a dependency on the hypervisor's
representation of 'bool', but I think such ought to be taken
care of in the function carrying out the write, not in the
macro here.

Jan

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