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

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Message-ID: <dee72388-764e-a123-13a8-3a47341c8c55@arm.com>
Date:   Thu, 24 Aug 2023 17:51:43 +0100
From:   James Morse <james.morse@....com>
To:     Reinette Chatre <reinette.chatre@...el.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 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.
|	 */


Thanks,

James