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Date: Tue, 24 Mar 2015 15:08:01 +0100
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Ingo Molnar <mingo@kernel.org>, Denys Vlasenko <vda.linux@googlemail.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 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss
References: <1427129240-15543-1-git-send-email-dvlasenk@redhat.com> <CALCETrW4n_PD=Y=Ozf8FGCHk_-+zvHHYCMXRTr4BmMG85ddQrQ@mail.gmail.com> <CALCETrXq7uxDCu5GCzVoyijuYQ9yoWkRbyPY1=rBFptgtWjQng@mail.gmail.com> <CAK1hOcMX+FuOyvmkTBr73n=sb5qi5i8rupyYNHLj9q0-ydDNtw@mail.gmail.com> <20150324063430.GB26302@gmail.com>
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On 03/24/2015 07:34 AM, Ingo Molnar wrote:
> 
> * Denys Vlasenko <vda.linux@googlemail.com> wrote:
> 
>> On Mon, Mar 23, 2015 at 9:38 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> Actually, I want to remove the added comment in the code.  I don't see
>>> why we should have a specific comment about SS and not about, say, CS,
>>> ESP, or anything else.  OK?
>>
>> Ok.
> 
> Might be nice to place a more generic description there, which 
> registers are expected to be saved by user-space calling in here, etc.

__kernel_vsyscall entry point has the same ABI in any 32-bit vDSO,
the good old int 0x80 calling convention:

syscall# in eax,
params in ebx/ecx/edx/esi/edi/ebp,
all registers are preserved by the syscall.

(I think we don't guarantee that all flags are preserved:
I have a testcase where DF gets cleared).

Each flavor of fast kernel call does necessary massaging to
conform to the ABI. E.g. SYSCALL-based fast call clobbers ecx,
so its vDSO saves/restores ecx on stack.

Do you want a patch which adds such comment into every vDSO?