linux-kernel - Re: [PATCH 3/4] x86: open-code register save/restore in trace

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Date:	Sat, 10 Jan 2015 21:14:03 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	X86 ML <x86@...nel.org>, Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 3/4] x86: open-code register save/restore in
 trace_hardirqs thunks

On Sat, Jan 10, 2015 at 3:23 PM, Borislav Petkov <bp@...en8.de> wrote:
> Bah, I see it. This nasty '$' gets forgotten a lot, maybe we should have
> a check for that in some scripts :-)
>
> Here's the fix:
>
> ---
> Index: b/arch/x86/lib/thunk_64.S
> ===================================================================
> --- a/arch/x86/lib/thunk_64.S   2015-01-10 15:18:04.418737613 +0100
> +++ b/arch/x86/lib/thunk_64.S   2015-01-10 15:17:18.882736556 +0100
> @@ -67,7 +67,7 @@ restore:
>         movq_cfi_restore 6*8, rdx
>         movq_cfi_restore 7*8, rsi
>         movq_cfi_restore 8*8, rdi
> -       addq 9*8, %rsp
> +       addq $9*8, %rsp
>         CFI_ADJUST_CFA_OFFSET -9*8
>         ret

Thanks!

After I've seen the disassembly I myself posted, I can't help but wonder
why we use 5-byte instructions to store and load regs on stack when
pushes and pops are 1 or 2-byte long.

Especially that 32-bit code *does* use push/pops.

Can you test the attached patch with your kvm guest testcase?

View attachment "0003-x86-open-code-register-save-restore-in-trace_hardirq.patch" of type "text/x-patch" (3038 bytes)