linux-kernel - Re: [PATCH v2 4/4] x86/fpu: Correct the legacy state offset and size information

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Date:   Wed, 28 Sep 2022 15:16:42 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
        <dave.hansen@...ux.intel.com>, <hpa@...or.com>, <avagin@...il.com>
Subject: Re: [PATCH v2 4/4] x86/fpu: Correct the legacy state offset and size
 information

On 9/28/2022 2:06 PM, Sean Christopherson wrote:
> On Thu, Sep 22, 2022, Chang S. Bae wrote:
>>
>> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
>> index a3f7045d1f8e..ac2ec5d6e7e4 100644
>> --- a/arch/x86/kernel/fpu/xstate.c
>> +++ b/arch/x86/kernel/fpu/xstate.c
>> @@ -143,8 +143,13 @@ static unsigned int xfeature_get_offset(u64 xcomp_bv, int xfeature)
>>   	 * offsets.
>>   	 */
>>   	if (!cpu_feature_enabled(X86_FEATURE_XCOMPACTED) ||
>> -	    xfeature <= XFEATURE_SSE)
>> +	    xfeature <= XFEATURE_SSE) {
>> +		if (xfeature <= XFEATURE_SSE)
>> +			pr_warn("The legacy state (%d) is discontiguously located.\n",
>> +				xfeature);
> 
> pr_warn() here isn't warranted.  copy_uabi_to_xstate() calls this with non-extended
> features, 

I think patch1 makes changes not to call this for legacy features anymore.

> which is perfectly fine since it manually handles MXCSR.  And that helper
> is directly reachable by userspace, i.e. userspace can spam the pr_warn().

I don't think I get your point. I assume that helper is 
__raw_xsave_addr(). But then I'm missing how it can be directly reached 
by userspace.

Thanks,
Chang