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SubjectRe: [PATCH Bugfix v2 2/4] x86/xsaves: Define and use user_xstate_size for xstate size in signal context
On 04/22/2015 12:05 PM, Yu, Fenghua wrote:
>> From: Hansen, Dave
>> Sent: Wednesday, April 22, 2015 11:45 AM
>> To: Yu, Fenghua; H. Peter Anvin; Ingo Molnar; Thomas Gleixner; Mallick, Asit
>> K; Williamson, Glenn P
>> Cc: linux-kernel; x86
>> Subject: Re: [PATCH Bugfix v2 2/4] x86/xsaves: Define and use
>> user_xstate_size for xstate size in signal context
>>
>> On 04/21/2015 09:51 PM, Fenghua Yu wrote:
>>> + /*
>>> + * Copy rest of xstates in compact format to user.
>>> + */
>>> + for (i = 2; i < xstate_features; i++) {
>>> + if (test_bit(i, (unsigned long *)&pcntxt_mask)) {
>>> + int user_offset, kernel_offset;
>>> + int size;
>>> +
>>> + user_offset = xstate_offsets[i];
>>> + kernel_offset = xstate_comp_offsets[i];
>>> + size = xstate_sizes[i];
>>> +
>>> + if (__copy_to_user(buf_fx + user_offset,
>>> + xsave + kernel_offset, size))
>>> + return -1;
>>> + }
>>> + }
>>
>> Is this checking the right bitmap?
>>
>> The 'xsaves' documentation says: "If RFBM[i] = 1, XSTATE_BV[i] is set to the
>> value of XINUSE[i]". Where "XINUSE denotes the state-component bitmap
>> corresponding to the init optimization".
>>
>> So shouldn't this be checking xsave->xsave_hdr.xstate_bv instead of
>> pcntxt_mask? The will be equal unless the "init optimization" is in play.
>
> Xsave->xsave_hdr.xstate_bv is equal to pcntxt_mask (see setup_init_fpu_buf()).

No. I don't think it works this way. xsaves *WRITES* to the
XSTATE_BV[] field in addition to reading it. According to the SDM:
"execution of XSAVES writes the XSTATE_BV field of the XSAVE header (see
Section 13.4.2), setting XSTATE_BV[i] (0 ≤ i ≤ 63) as follows...
Execution of XSAVES performs the init optimization to reduce the amount
of data written to memory. If XINUSE[i] = 0, state component i is not
saved to the XSAVE area (even if RFBM[i] = 1)".

> And here XINUSE and RFBM are irrelevant because they are used in init or modified
> optimization during xsave*/xrstore*. We only copy out the xstates from kernel
> to user and XINUSE and RFBM are not in scope here.

I think they are in use. The kernel created its buffer with 'xsaves' so
when it is reading the buffer it needs to take those optimizations in to
consideration.




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