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>,
Juergen Gross <jgross@suse.com>, 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 V6 3/8] x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()
Date: Thu, 28 Apr 2022 12:26:37 +0200 [thread overview]
Message-ID: <Ympr3VU5Q182SVT7@zn.tnic> (raw)
In-Reply-To: <CAJhGHyANWhu-HX20_4XhgACE8tUd8SFa5BTH-ynJscNQ7QxBRw@mail.gmail.com>
On Thu, Apr 28, 2022 at 08:33:36AM +0800, Lai Jiangshan wrote:
> The branch in sync_regs() can be optimized out for the non-XENPV case
> since XENPV doesn't call sync_regs() after patch5 which makes XENPV
> not call error_entry().
I find it confusing why you're even mentioning optimizing a simple test.
That thing is basically a *whopping* two fastpath instructions:
# arch/x86/kernel/traps.c:853: if (regs != eregs)
cmpq %rdi, %rax # eregs, <retval>
je .L255 #,
Is this such a hot path that it shows in some profiles and removing that
test there is so important?
> The aim of this patch and most of the patchset is to make
> error_entry() be able to be converted to C. And XENPV cases can
> also be optimized in the patchset although it is not the major main.
So say just that.
> Since this patch adds extra overhead (CALL and RET), the changelog
> has to explain why it is worth it not just for converting ASM to C.
What is the practical relevance of this very minor overhead? Does it
show even in a microbenchmark?
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2022-04-28 10:26 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-21 14:10 [PATCH V6 0/8] x86/entry: Clean up entry code Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 1/8] x86/traps: Move pt_regs only in fixup_bad_iret() Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 2/8] x86/entry: Switch the stack after error_entry() returns Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 3/8] x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry() Lai Jiangshan
2022-04-27 17:45 ` Borislav Petkov
2022-04-28 0:33 ` Lai Jiangshan
2022-04-28 10:26 ` Borislav Petkov [this message]
2022-05-02 12:42 ` Juergen Gross
2022-05-02 18:09 ` Borislav Petkov
2022-04-21 14:10 ` [PATCH V6 4/8] x86/entry: Move cld to the start of idtentry macro Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 5/8] x86/entry: Don't call error_entry() for XENPV Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 6/8] x86/entry: Convert SWAPGS to swapgs and remove the definition of SWAPGS Lai Jiangshan
2022-04-29 9:56 ` Borislav Petkov
2022-04-29 11:45 ` Lai Jiangshan
2022-04-29 12:22 ` Borislav Petkov
2022-05-02 12:18 ` Juergen Gross
2022-05-02 17:56 ` Borislav Petkov
2022-05-03 10:43 ` Borislav Petkov
2022-04-21 14:10 ` [PATCH V6 7/8] x86/entry: Remove the branch in sync_regs() Lai Jiangshan
2022-04-21 14:10 ` [PATCH V6 8/8] x86/entry: Use idtentry macro for entry_INT80_compat() Lai Jiangshan
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