linux-kernel - Re: [PATCH 15/22] x86/entry: Add TIF_NEED_FPU

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Message-ID: <20190207114941.jzydrry5xpjngyiv@linutronix.de>
Date:   Thu, 7 Feb 2019 12:49:42 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 15/22] x86/entry: Add TIF_NEED_FPU_LOAD

On 2019-01-30 12:55:07 [+0100], Borislav Petkov wrote:
> This definitely needs to be written somewhere in
> 
> arch/x86/include/asm/fpu/internal.h
> 
> or where we decide to put the FPU handling rules.

Added:

Index: staging/arch/x86/include/asm/fpu/internal.h
===================================================================
--- staging.orig/arch/x86/include/asm/fpu/internal.h
+++ staging/arch/x86/include/asm/fpu/internal.h
@@ -537,6 +537,12 @@ static inline void __fpregs_load_activat
  *
  * The FPU context is only stored/restore for user task and ->mm is used to
  * distinguish between kernel and user threads.
+ *
+ * If TIF_NEED_FPU_LOAD is cleared then CPU's FPU registers are holding the
+ * current content of current()'s FPU register state.
+ * If TIF_NEED_FPU_LOAD is set then CPU's FPU registers may not hold current()'s
+ * FPU registers. It is required to load the register before returning to
+ * userland or using the content otherwise.
  */
 static inline void
 switch_fpu_prepare(struct fpu *old_fpu, int cpu)

Sebastian