linux-kernel - Re: [PATCH 08/22] x86/fpu: Remove user_fpu

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Message-ID: <20190205181601.sshzxcj3wv65vhfm@linutronix.de>
Date:   Tue, 5 Feb 2019 19:16:01 +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 08/22] x86/fpu: Remove user_fpu_begin()

On 2019-01-25 16:18:40 [+0100], Borislav Petkov wrote:
> Reviewed-by: Borislav Petkov <bp@...e.de>
thanks.

> Should we do this microoptimization in addition, to save us the
> activation when the kernel thread here:
> 
> 	taskA -> kernel thread -> taskA
> 
> doesn't call kernel_fpu_begin() and thus fpu_fpregs_owner_ctx remains
> the same?

This might work now but at the end of the series this case will be
handled. The switch
	taskA -> kernel thread

will save taskA's registers. The switch
	kernel thread -> taskA

will only set TF flag to restore FPU registers on the return to
userland. The load happens only the ctx pointer is different.

> It would be a bit more correct as it won't invoke the
> trace_x86_fpu_regs_activated() TP in case the FPU context is the same.

The trace point is not wrong. As of now the same context will be loaded
again.

Sebastian