linux-kernel - Re: [Xen-devel] [PATCH] xen: don't use privcmd_call() from xen_mc

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Date:   Wed, 13 Jun 2018 06:55:21 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Juergen Gross" <jgross@...e.com>
Cc:     "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen: don't use privcmd_call() from
 xen_mc_flush()

>>> On 13.06.18 at 13:12, <jgross@...e.com> wrote:
> On 13/06/18 12:20, Jan Beulich wrote:
>>>>> On 13.06.18 at 12:05, <JBeulich@...e.com> wrote:
>>>>>> On 13.06.18 at 11:58, <jgross@...e.com> wrote:
>>>> Using privcmd_call() for a singleton multicall seems to be wrong, as
>>>> privcmd_call() is using stac()/clac() to enable hypervisor access to
>>>> Linux user space.
>>>>
>>>> Add a new xen_single_call() function to be use for that purpose.
>>>>
>>>> Reported-by: Jan Beulich <jbeulich@...e.com>
>>>> Signed-off-by: Juergen Gross <jgross@...e.com>
>>>
>>> Reviewed-by: Jan Beulich <jbeulich@...e.com>
>> 
>> Actually I've only now realized that this isn't a real problem right now:
>> PV can't use SMAP (we don't provide a virtualized version of it), and
>> HVM/PVH can't use multicalls (which may have to change for PVH Dom0,
>> so having the change in place is helpful anyway), so the whole
>> in-kernel logic to collect and issue batches should be unreachable there.
>> 
>> But perhaps the commit message would benefit from a little bit of
>> re-wording.
> 
> Hmm, right.
> 
> What about:
> 
> "Using privcmd_call() for a singleton multicall seems to be wrong, as
>  privcmd_call() is using stac()/clac() to enable hypervisor access to
>  Linux user space.
> 
>  Even if currently not a problem (pv domains can't use SMAP while HVM
>  and PVH domains can't use multicalls) things might change when
>  PVH dom0 support is added to the kernel."

SGTM

Jan