Re: [PATCH] libs/guest: Don't hide the indirection on xc_cpu_policy_t

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
	Jan Beulich <JBeulich@suse.com>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH] libs/guest: Don't hide the indirection on xc_cpu_policy_t
Date: Wed, 5 May 2021 11:16:08 +0200	[thread overview]
Message-ID: <YJJiWLqGoHLSnj01@Air-de-Roger> (raw)
In-Reply-To: <20210504185322.19306-1-andrew.cooper3@citrix.com>

On Tue, May 04, 2021 at 07:53:22PM +0100, Andrew Cooper wrote:
> It is bad form in C, perhaps best demonstrated by trying to read
> xc_cpu_policy_destroy(), and causes const qualification to have
> less-than-obvious behaviour (the hidden pointer becomes const, not the thing
> it points at).

Would this also affect cpuid_leaf_buffer_t and msr_entry_buffer_t
which hide an array behind a typedef?

> xc_cpu_policy_set_domain() needs to drop its (now normal) const qualification,
> as the policy object is modified by the serialisation operation.
> 
> This also shows up a problem with the x86_cpu_policies_are_compatible(), where
> the intermediate pointers are non-const.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

> ---
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Roger Pau Monné <roger.pau@citrix.com>
> CC: Wei Liu <wl@xen.org>
> 
> Discovered while trying to start the integration into XenServer.  This wants
> fixing ASAP, before futher uses get added.
> 
> Unsure what to do about x86_cpu_policies_are_compatible().  It would be nice
> to have xc_cpu_policy_is_compatible() sensibly const'd, but maybe that means
> we need a struct const_cpu_policy and that smells like it is spiralling out of
> control.

Not sure TBH, I cannot think of any alternative right now, but
introducing a const_cpu_policy feels kind of code duplication.

Thanks, Roger.