linux-kernel - Re: [PATCH 04/11] x86/fpu: eager switch PKRU state

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Message-ID: <642da544-908f-74e5-6168-10acff39f3f3@linux.intel.com>
Date:   Thu, 18 Oct 2018 10:50:38 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH 04/11] x86/fpu: eager switch PKRU state

>>> diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
>>> index 19b137f1b3beb..b184f916319e5 100644
>>> --- a/arch/x86/include/asm/pkeys.h
>>> +++ b/arch/x86/include/asm/pkeys.h
>>> @@ -119,7 +119,7 @@ extern int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
>>>  		unsigned long init_val);
>>>  extern int __arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
>>>  		unsigned long init_val);
>>> -extern void copy_init_pkru_to_fpregs(void);
>>> +extern void pkru_set_init_value(void);
>>
>> Could you elaborate on why the name is being changed?
> 
> The function name read like init_pkru value is copied to fpregs save
> area which is not the case. I could revert it if you prefer.

I would just prefer that we find some way of saying *where* pkru is
being set.  Is it fpstate, fpregs, or both?

>>> +	 * switch between those two operation would load the new value from the
>>> +	 * updated xstate and then we would write (the same value) to the CPU.
>>> +	 */
>>> +	pk->pkru = pkru;
>>> +	__write_pkru(pkru);
>>> +
>>> +}
>>
>> There's an unnecessary line there.
>>
>> This also needs a lot more high-level context about why it is necessary.
>>  I think you had that in the changelog, but we also need the function
>> commented.
> 
> What is necessary? The manual update of "pk->pkru?

Why we need to update fpstate *and* the actual register.