linux-kernel - Re: [PATCH 01/17] x86, fpu: wrap get_xsave

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Date:	Tue, 24 Mar 2015 21:01:44 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave.hansen@...el.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Oleg Nesterov <oleg@...hat.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 01/17] x86, fpu: wrap get_xsave_addr() to make it safer

On 03/24/2015 08:18 PM, Andy Lutomirski wrote:
> On Tue, Mar 24, 2015 at 5:12 PM, Dave Hansen <dave.hansen@...el.com> wrote:

>> I won't completely claim to understand what's going on with the FPU
>> code, but I think your analysis is a bit off.
>>
>> unlazy_fpu() does __save_init_fpu() which (among other things) calls
>> xsave to dump the CPU registers to memory.  That doesn't make any sense
>> to do if "The in-memory copy and the CPU regs match."
>>
>> IOW, unlazy_fpu() is called when the in-memory copy is garbage and takes
>> us to a state where we can look at the in-memory copy.
> 
> I think that __save_init_fpu (called by unlazy_fpu) does that, but
> __thread_fpu_end calls __thread_clear_has_fpu, which, in turn, zaps
> fpu_owner_task, which will force an unnecessary xrstor.  Or maybe not
> if we have further bugs.

Indeed, __save_init_fpu (yeah, terrible name) will save
the in-register state to memory for you, so you can
inspect it.

Is there any reason not to rename __save_init_fpu to
save_fpu_state, or just save_fpu?

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