Re: [v4 11/17] vt-d: Add API to update IRTE when VT-d PI is used

From: "Jan Beulich" <JBeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>,
	Feng Wu <feng.wu@intel.com>,
	xen-devel@lists.xen.org
Cc: Yang Zhang <yang.z.zhang@intel.com>,
	Kevin Tian <kevin.tian@intel.com>, Keir Fraser <keir@xen.org>
Subject: Re: [v4 11/17] vt-d: Add API to update IRTE when VT-d PI is used
Date: Thu, 23 Jul 2015 09:52:09 -0600	[thread overview]
Message-ID: <55B129C90200007800094C1D@prv-mh.provo.novell.com> (raw)
In-Reply-To: <55B0F17D.1060702@citrix.com>

>>> On 23.07.15 at 15:51, <andrew.cooper3@citrix.com> wrote:
> On 23/07/15 12:35, Feng Wu wrote:
>> +    GET_IREMAP_ENTRY(ir_ctrl->iremap_maddr, remap_index, iremap_entries, p);
>> +
>> +    old_ire = new_ire = *p;
>> +
>> +    /* Setup/Update interrupt remapping table entry. */
>> +    setup_posted_irte(&new_ire, pi_desc, gvec);
>> +    ret = cmpxchg16b(p, &old_ire, &new_ire);
>> +
>> +    ASSERT(ret == *(__uint128_t *)&old_ire);
> 
> This cannot be correct.  Either the cmpxchg() is required and you must
> cope with it failing, or the cmpxchg() is not required and this should
> be a plain write.

Not exactly: The cmpxchg() is required for this to be an atomic
128-bit write. And hence I would view the ASSERT() as
appropriate - it validates that the entry didn't change behind our
back.

Jan