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Date: Tue, 10 Mar 2015 14:28:45 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, 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 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64
 fastpath
Message-ID: <20150310132845.GA26490@gmail.com>
References: <1425926364-9526-1-git-send-email-dvlasenk@redhat.com>
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 <20150310125151.GB21686@gmail.com>
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 <20150310132147.GB26185@gmail.com>
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> * Denys Vlasenko <dvlasenk@redhat.com> wrote:
> 
> > > So there are now +2 instructions (5 instead of 3) in the 
> > > system_call path, but there are -2 instructions in the SYSRETQ 
> > > path,
> > 
> > Unfortunately, no. [...]
> 
> So I assumed that it was an equivalent transformation, given that 
> none of the changelogs spelled out the increase in overhead ...

Also, for future reference, you need to spell out performance costs of 
fast path patches very clearly - and that wasn't done here.

That's a big no-no, please don't do it again ...

Thanks,

	Ingo