linux-kernel - Re: [PATCH 3/3] x86: avoid copying dynamic FP state from init

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Message-ID: <Z78Vt8yCcPrFQeqo@gmail.com>
Date: Wed, 26 Feb 2025 14:23:03 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Benjamin Berg <benjamin@...solutions.net>
Cc: linux-arch@...r.kernel.org, linux-um@...ts.infradead.org,
	x86@...nel.org, briannorris@...omium.org,
	linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [PATCH 3/3] x86: avoid copying dynamic FP state from init_task


* Benjamin Berg <benjamin@...solutions.net> wrote:

> > Note that this patch, while it still applies cleanly, crashes/hangs 
> > the x86-64 defconfig kernel bootup in the early boot phase in a KVM 
> > guest bootup.
> 
> Oh, outch. It seems that arch_task_struct_size can actually become 
> smaller than sizeof(init_task) if the CPU does not have certain 
> features.
> 
> See fpu__init_task_struct_size, which does:
> 
>   int task_size = sizeof(struct task_struct);
>   task_size -= sizeof(current->thread.fpu.__fpstate.regs);
>   task_size += fpu_kernel_cfg.default_size;
> 
> I'll submit a new version of the patch and then also switch to use
> memcpy_and_pad.

Thank you!

	Ingo