Messages in this thread | | | Date | Wed, 22 Apr 2015 12:34:02 -0700 | From | Dave Hansen <> | Subject | Re: [PATCH Bugfix v2 2/4] x86/xsaves: Define and use user_xstate_size for xstate size in signal context |
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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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