linux-kernel - Re: [PATCH V7 05/18] x86/pks: Add PKS setup code

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Date:   Fri, 26 Nov 2021 11:11:36 +0800
From:   "taoyi.ty" <escape@...ux.alibaba.com>
To:     Thomas Gleixner <tglx@...utronix.de>, ira.weiny@...el.com,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-mm@...ck.org
Subject: Re: [PATCH V7 05/18] x86/pks: Add PKS setup code

On 11/25/21 11:15 PM, Thomas Gleixner wrote:
> On Tue, Aug 03 2021 at 21:32, ira weiny wrote:
>> +#ifdef CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS
>> +
>> +void setup_pks(void);
> 
> pks_setup()
> 
>> +#ifdef CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS
>> +
>> +static DEFINE_PER_CPU(u32, pkrs_cache);
>> +u32 __read_mostly pkrs_init_value;
>> +
>> +/*
>> + * write_pkrs() optimizes MSR writes by maintaining a per cpu cache which can
>> + * be checked quickly.
>> + *
>> + * It should also be noted that the underlying WRMSR(MSR_IA32_PKRS) is not
>> + * serializing but still maintains ordering properties similar to WRPKRU.
>> + * The current SDM section on PKRS needs updating but should be the same as
>> + * that of WRPKRU.  So to quote from the WRPKRU text:
>> + *
>> + *     WRPKRU will never execute transiently. Memory accesses
>> + *     affected by PKRU register will not execute (even transiently)
>> + *     until all prior executions of WRPKRU have completed execution
>> + *     and updated the PKRU register.
>> + */
>> +void write_pkrs(u32 new_pkrs)
> 
> pkrs_write()
> 
>> +{
>> +	u32 *pkrs;
>> +
>> +	if (!static_cpu_has(X86_FEATURE_PKS))
>> +		return;
> 
>    cpu_feature_enabled() if at all. Why is this function even invoked
>    when PKS is off?
> 
>> +
>> +	pkrs = get_cpu_ptr(&pkrs_cache);
> 
> As far as I've seen this is mostly called from non-preemptible
> regions. So that get/put pair is pointless. Stick a lockdep assert into
> the code and disable preemption at the maybe one callsite which needs
> it.
> 
>> +	if (*pkrs != new_pkrs) {
>> +		*pkrs = new_pkrs;
>> +		wrmsrl(MSR_IA32_PKRS, new_pkrs);
>> +	}
>> +	put_cpu_ptr(pkrs);
>> +}
>> +
>> +/*
>> + * Build a default PKRS value from the array specified by consumers
>> + */
>> +static int __init create_initial_pkrs_value(void)
>> +{
>> +	/* All users get Access Disabled unless changed below */
>> +	u8 consumer_defaults[PKS_NUM_PKEYS] = {
>> +		[0 ... PKS_NUM_PKEYS-1] = PKR_AD_BIT
>> +	};
>> +	int i;
>> +
>> +	consumer_defaults[PKS_KEY_DEFAULT] = PKR_RW_BIT;
>> +
>> +	/* Ensure the number of consumers is less than the number of keys */
>> +	BUILD_BUG_ON(PKS_KEY_NR_CONSUMERS > PKS_NUM_PKEYS);
>> +
>> +	pkrs_init_value = 0;
> 
> This needs to be cleared because the BSS might be non zero?
> 
>> +	/* Fill the defaults for the consumers */
>> +	for (i = 0; i < PKS_NUM_PKEYS; i++)
>> +		pkrs_init_value |= PKR_VALUE(i, consumer_defaults[i]);
> 
> Also PKR_RW_BIT is a horrible invention:
> 
>> +#define PKR_RW_BIT 0x0
>>   #define PKR_AD_BIT 0x1
>>   #define PKR_WD_BIT 0x2
>>   #define PKR_BITS_PER_PKEY 2
> 
> This makes my brain spin. How do you fit 3 bits into 2 bits per key?
> That's really non-intuitive.
> 
> PKR_RW_ENABLE		0x0
> PKR_ACCESS_DISABLE	0x1
> PKR_WRITE_DISABLE	0x2
> 
> makes it obvious what this is about, no?
> 
> Aside of that, the function which set's up the init value is really
> bogus. As you explained in the cover letter a kernel user has to:
> 
>     1) Claim an index in the enum
>     2) Add a default value to the array in that function
> 
> Seriously? How is that any better than doing:
> 
> #define PKS_KEY0_DEFAULT	PKR_RW_ENABLE
> #define PKS_KEY1_DEFAULT	PKR_ACCESS_DISABLE
> ...
> #define PKS_KEY15_DEFAULT	PKR_ACCESS_DISABLE
> 
> and let the compiler construct pkrs_init_value?
> 
> TBH, it's not and this function has to be ripped out in case that you
> need a dynamic allocation of keys some day. So what is this buying us
> aside of horrible to read and utterly pointless code?
> 
>> +	return 0;
>> +}
>> +early_initcall(create_initial_pkrs_value);
>> +
>> +/*
>> + * PKS is independent of PKU and either or both may be supported on a CPU.
>> + * Configure PKS if the CPU supports the feature.
>> + */
>> +void setup_pks(void)
>> +{
>> +	if (!cpu_feature_enabled(X86_FEATURE_PKS))
>> +		return;
>> +
>> +	write_pkrs(pkrs_init_value);
> 
> Is the init value set up _before_ this function is invoked for the first
> time?
> 
> Thanks,
> 
>          tglx
> 
Setting up for cpu0 is before create_initial_pkrs_value. therefore pkrs 
value of cpu0 won't be set correctly.

[root@...Yun ~]# rdmsr -a 0x000006E1
0
55555554
55555554
55555554
55555554
55555554
55555554
55555554
55555554
55555554

Here are my test results after applying the patches

Thanks.