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

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Message-ID: <Z8iUMRGyUpgisuc_@gmail.com>
Date: Wed, 5 Mar 2025 19:13:05 +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 09:37, Ingo Molnar wrote:
> > 
> > * 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().
> 
> That's true. But wouldn't it be a bit silly to include _all_ FPU
> registers? If the kernel isn't using AVX512, why bother saving and
> restoring AVX512?

Fair enough - although I bet the execution time difference between a 
partial and a full FPU context save isn't as large as the buffer size 
would suggest... There's a lot of setup cost in XSAVE* instructions 
last I checked.

Thanks,

	Ingo