linux-kernel - Re: [PATCH v5 02/24] x86/resctrl: Access per-rmid structures by index

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Date:   Thu, 24 Aug 2023 17:29:39 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     James Morse <james.morse@....com>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>
CC:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        <shameerali.kolothum.thodi@...wei.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        <carl@...amperecomputing.com>, <lcherian@...vell.com>,
        <bobo.shaobowang@...wei.com>, <tan.shaopeng@...itsu.com>,
        <xingxin.hx@...nanolis.org>, <baolin.wang@...ux.alibaba.com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>, <peternewman@...gle.com>,
        <dfustini@...libre.com>
Subject: Re: [PATCH v5 02/24] x86/resctrl: Access per-rmid structures by index

Hi James,

On 8/24/2023 9:51 AM, James Morse wrote:
> Hi Reinette,
> 
> On 09/08/2023 23:32, Reinette Chatre wrote:
>> On 7/28/2023 9:42 AM, James Morse wrote:
>>> @@ -377,14 +399,17 @@ static void add_rmid_to_limbo(struct rmid_entry *entry)
>>>  
>>>  void free_rmid(u32 closid, u32 rmid)
>>>  {
>>> +	u32 idx = resctrl_arch_rmid_idx_encode(closid, rmid);
>>>  	struct rmid_entry *entry;
>>>  
>>> -	if (!rmid)
>>> -		return;
>>> -
>>>  	lockdep_assert_held(&rdtgroup_mutex);
>>>  
>>> -	entry = __rmid_entry(closid, rmid);
>>> +	/* do not allow the default rmid to be free'd */
>>> +	if (idx == resctrl_arch_rmid_idx_encode(RESCTRL_RESERVED_CLOSID,
>>> +						RESCTRL_RESERVED_RMID))
>>> +		return;
>>> +
> 
>> Why is this encoding necessary? Can the provided function parameters
>> not be tested directly against RESCTRL_RESERVED_CLOSID and
>> RESCTRL_RESERVED_RMID ?
> 
> Doing this by encoding means if the architecture code supplies an
> resctrl_arch_rmid_idx_encode() that ignores the closid, this reduces down to:
> | if (rmid == RESCTRL_RESERVED_RMID)
> 
> which is what the code did before. I'll add a comment:
> |	/*
> |	 * Do not allow RESCTRL_RESERVED_RMID to be free'd. Comparing by index
> |	 * allows architectures that ignore the closid parameter to avoid an
> |	 * unnecessary check.
> |	 */
> 

Sounds good. Thank you.

Reinette