Re: [PATCH V5 3/7] x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()

From: Borislav Petkov <bp@alien8.de>
To: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
	Lai Jiangshan <jiangshan.ljs@antgroup.com>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH V5 3/7] x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()
Date: Tue, 12 Apr 2022 16:30:58 +0200	[thread overview]
Message-ID: <YlWNIj198mKM3hMG@zn.tnic> (raw)
In-Reply-To: <CAJhGHyAsPZAeXUoOa5LJ5bxuMOWzg=cvYNYPVT9kpBM7Tsf3xA@mail.gmail.com>

On Tue, Apr 12, 2022 at 09:52:44PM +0800, Lai Jiangshan wrote:
> The mapped size for the text is always 2M when the kernel is booted
> since it is 2M-aligned.  So I don't think the size is a concern.

This is not how this argumentation works: you add 0.01% size increase
here, 0.01% slowdown there and it all soon adds up into a bloated
kernel. Besides, nowadays the kernel runs a lot as a guest and guest
kernel size does matter.

So no, just because you want to turn error_entry() into C code and for
that you'll add some bloat doesn't justify it in my book. All changes to
core code need to be absolutely needed and justifiable.

> For example, a heavy page fault and IPI or timer at the same time.
> I'm not sure if it is a real case.

I wouldn't be surprised if it shows in some perf profiles.

> I'm Okay with asm_error_entry().  And also we can use an ASM function
> containing PUSH_AND_CLEAR_REGS only.

Just do the necessary and minimal thing - this is core code and in asm
for that matter, so be conservative pls. This is not some random driver.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette