Re: [PATCH 01/17] x86, fpu: wrap get_xsave_addr() to make it safer

Rik van Riel
2015-03-25T01:02:01Z

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