On 4/26/19 9:50 AM, Sebastian Andrzej Siewior wrote: > On 2019-04-26 09:33:28 [-0700], Dave Hansen wrote: >> On 4/26/19 12:26 AM, Sebastian Andrzej Siewior wrote: >>>> That's just a guess, though. >>>> >>>> If we care, I think we should just use XSAVE instead of XSAVEOPT and >>>> trying to reconstruct the init state in software. >>> We can't use XSAVE directly in the slowpath. We need to reconstruct the >>> init state. We have the mxcsr quirk. We would need just to extend it and >>> set the FP area to init state if the FP state is missing like we do in >>> fpstate_sanitize_xstate(). >> >> Can you remind me why we can't use XSAVE directly in the slow path? > > Where to? In the fastpath we XSAVE directly to task's stack. We are > in the slowpath because this failed. I'm looking at the code and having a bit of a hard time connecting it to what you're saying here. We are in copy_fpstate_to_sigframe(), right? Let's assume we are using_compacted_format(). If copy_fpregs_to_sigframe() fails to copy, we return immediately and never get to the save_xstate_epilog() code in question. So, to even get to save_xstate_epilog(), we had to do a *successful* copy_fpstate_to_sigframe() which, on XSAVE systems will use copy_xregs_to_user() which already uses plain XSAVE (not XSAVEOPT). save_xstate_epilog() goes and tries to set XFEATURE_MASK_FPSSE on this XSAVE (literally the XSAVE instruction) generated header. But, if we're dealing with header.xfeatures generated by an XSAVE with the RFBM=-1, I don't understand how header.xfeatures could ever *not* have XFEATURE_MASK_FPSSE set. The only way would be if we had gotten here after using FXSAVE, but I believe that's impossible since those systems have use_xsave()==0. IOW, I think the patch is right, but I'm not sure I totally agree with the reasoning.
