linux-kernel - Re: [RFC PATCH v2] x86/fpu: make kernel-mode FPU reliably usable in softirqs

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Message-ID: <Z8iL1dY3o9OxQgBy@gmail.com>
Date: Wed, 5 Mar 2025 18:37:25 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Eric Biggers <ebiggers@...nel.org>, x86@...nel.org,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ard Biesheuvel <ardb@...nel.org>,
	Ben Greear <greearb@...delatech.com>,
	Xiao Liang <shaw.leon@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	"Jason A . Donenfeld" <Jason@...c4.com>,
	"Bae, Chang Seok" <chang.seok.bae@...el.com>
Subject: Re: [RFC PATCH v2] x86/fpu: make kernel-mode FPU reliably usable in
 softirqs


* Dave Hansen <dave.hansen@...el.com> wrote:

> On 3/5/25 01:07, Ingo Molnar wrote:>> Alternatives considered:
> >> - Make kernel-mode FPU sections fully preemptible.  This would require
> >>   growing task_struct by another struct fpstate which is more than 2K.
> > 
> > So that's something that will probably happen once the kernel is built 
> > using APX anyway?
> 
> I was expecting that building the kernel with APX would be very 
> different than a kernel_fpu_begin(). We don't just need *one* more 
> save area for APX registers: we need a stack, just like normal GPRs.

Yes - but my point is: with any APX build we'd probably be saving 
FPU(-ish) registers at entry points, into a separate context area. If 
that includes FPU registers then we'd not have to do 
kernel_fpu_begin()/end().

In other words, we'd be doing something close to 'growing task_struct 
by another struct fpstate', or so - regardless of whether it's in 
task_struct or some sort of extended pt_regs. The kernel would also be 
close to 'FPU-safe', i.e. there likely wouldn't be a need for 
kernel_fpu_begin()/end().

Thanks,

	Ingo