linux-kernel - Re: [RFC PATCH] x86/fpu: Don't unconditionally add XFEATURE_MASK

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Date:   Thu, 25 Apr 2019 14:13:05 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, jannh@...gle.com, riel@...riel.com,
        mingo@...hat.com, bp@...e.de, Jason@...c4.com, luto@...nel.org,
        tglx@...utronix.de, rkrcmar@...hat.com, mingo@...nel.org,
        hpa@...or.com, kvm@...r.kernel.org, pbonzini@...hat.com,
        kurt.kanzenbach@...utronix.de
Subject: Re: [RFC PATCH] x86/fpu: Don't unconditionally add
 XFEATURE_MASK_FPSSE on sigentry

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.