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Date: Sun, 21 Aug 2011 21:05:43 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andrew Lutomirski <luto@mit.edu>, Borislav Petkov <bp@amd64.org>
CC: Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>, mingo@redhat.com,
        Richard Weinberger <richard@nod.at>,
        user-mode-linux-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org
Subject: Re: SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird
 crap with vdso on uml/i386)
References: <20110821063443.GH2203@ZenIV.linux.org.uk> <20110821084230.GI2203@ZenIV.linux.org.uk> <CAObL_7GXyzAzNjBtgye9BwXLyHm337oBTdjRDjbSPqoKZGWyyA@mail.gmail.com> <20110821144352.GJ2203@ZenIV.linux.org.uk> <20110821164124.GL2203@ZenIV.linux.org.uk> <CAObL_7E1vU8+0UQST6Smf9j40TckTzB=QdY0AuAWo4AX6cs2xg@mail.gmail.com> <20110822011645.GM2203@ZenIV.linux.org.uk> <CA+55aFz1jCZGcQ-c6uGN=k8nKDuGoz5g8e+pxpYAg4X_p7=5Mw@mail.gmail.com> <4E51B56F.3080301@zytor.com> <CAObL_7HLDD=FVSN58W7a0g0R--8x7OVrQVCBxWD1DirVZxRn2w@mail.gmail.com> <20110822020737.GP2203@ZenIV.linux.org.uk> <CAObL_7HY7qFUoqtNju42=BHTR8WuuW-cL_cQh_uUfx_7gFfHRg@mail.gmail.com>
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Borislav,

We're tracking down an issue with the way system call arguments are
handled on 32 bits.  We have a solution for SYSENTER but not SYSCALL;
fixing SYSCALL "properly" appears to be very difficult at best.

So the question is: how much overhead would it be to simply fall back to
int $0x80 or some other legacy-style domain crossing instruction for
32-bit system calls on AMD64 processors?  We don't ever use SYSCALL in
legacy mode, so native i386 kernels are unaffected.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.