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Date: Tue, 17 Mar 2015 14:51:28 +0100
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Ingo Molnar <mingo@kernel.org>
CC: Denys Vlasenko <dvlasenk@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] x86/asm/entry/64: Enable interrupts *after* we fetch
 PER_CPU_VAR(old_rsp)
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On 03/17/2015 02:42 PM, Denys Vlasenko wrote:
> Without this change, it is still not possible to get rid of
> PER_CPU_VAR(old_rsp) usage in switch_to: if preemption happens
> while we did not fetch PER_CPU_VAR(old_rsp) and stored it in pt_regs->sp,
> PER_CPU_VAR(old_rsp) gets corrupted by other task's user sp.
> 
> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Steven Rostedt <rostedt@goodmis.org>
> CC: Ingo Molnar <mingo@kernel.org>
> CC: Borislav Petkov <bp@alien8.de>
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Andy Lutomirski <luto@amacapital.net>
> CC: Oleg Nesterov <oleg@redhat.com>
> CC: Frederic Weisbecker <fweisbec@gmail.com>
> CC: Alexei Starovoitov <ast@plumgrid.com>
> CC: Will Drewry <wad@chromium.org>
> CC: Kees Cook <keescook@chromium.org>
> CC: x86@kernel.org
> CC: linux-kernel@vger.kernel.org
> ---
> 
> Run-tested, including with PARAVIRT on.

Well. The testing with PARAVIRT did work, however, I don't know why.

>  	movq_cfi rax,ORIG_RAX
> +	/*
> +	 * No need to follow this irqs off/on section - it's straight
> +	 * and short:
> +	 */
> +	ENABLE_INTERRUPTS(CLBR_RAX)

Here I wrongly assumed that now I can clobber rax, since it is saved
in pt_regs->orig_ax now. Wrong. Code below still wants to use rax
register directly.

Looks like I was "lucky" and paravirt call happen to not change rax.
I'll send a v2 patch without this ill-advised attempt of optimization.
Sorry.