Re: [RFC PATCH] x86/fpu: Don't unconditionally add XFEATURE_MASK_FPSSE on sigentry

[Dave Hansen]

On 4/25/19 10:35 AM, Sebastian Andrzej Siewior wrote:
> This commit reverts commit 04944b793e18e ("x86: xsave: set FP, SSE bits
> in the xsave header in the user sigcontext"). The commit claims that it
> is required for legacy applications but fails to explain why this is
> needed and it is not obvious to me why the application would require the
> FP/SSE state in the signal handler.

Any software that understands XSAVE is OK.  I think the legacy software
would be that which groks 'fxregs_state, and FXSAVE/FXRSTOR but does not
comprehend XSAVE/XRSTOR.  *That* software might change fxregs_state in
the signal frame, but the lack of XFEATURE_MASK_FPSSE in xfeatures would
prevent XRSTOR from restoring it.

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.