linux-kernel - Re: [PATCH 7/8] Add 32 bit VDSO time support for 32 bit kernel

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Date:	Tue, 04 Feb 2014 07:25:34 +0100
From:	Stefani Seibold <stefani@...bold.net>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	andriy.shevchenko@...ux.intel.com, Martin.Runge@...de-schwarz.com,
	Andreas.Brief@...de-schwarz.com
Subject: Re: [PATCH 7/8] Add 32 bit VDSO time support for 32 bit kernel

Am Montag, den 03.02.2014, 14:04 -0800 schrieb Andy Lutomirski:
> On Mon, Feb 3, 2014 at 2:01 PM, Stefani Seibold <stefani@...bold.net> wrote:
> > Am Montag, den 03.02.2014, 08:36 -0800 schrieb Andy Lutomirski:
> >> On Sun, Feb 2, 2014 at 11:44 PM, Stefani Seibold <stefani@...bold.net> wrote:
> >> > Am Sonntag, den 02.02.2014, 16:12 -0800 schrieb Andy Lutomirski:
> >> >> On Sun, Feb 2, 2014 at 1:39 PM, Stefani Seibold <stefani@...bold.net> wrote:
> >> >> > Am Sonntag, den 02.02.2014, 08:46 -0800 schrieb Andy Lutomirski:
> >> >> >> On Sun, Feb 2, 2014 at 3:27 AM,  <stefani@...bold.net> wrote:
> >> >> >> > From: Stefani Seibold <stefani@...bold.net>
> >> >> >> >
> >> >> >> > This patch add the time support for 32 bit a VDSO to a 32 bit kernel.
> >> >> >>
> >> >> >> [...]
> >> >> >>
> >> >> >> Can you address the review comments from last time around?  For
> >> >> >> example, this still seems to have redundant vvar and hpet mappings, it
> >> >> >> doesn't use the VVAR macro, it moves the 32-bit compat vDSO, etc.
> >> >> >>
> >> >> >
> >> >> > I will address the compat VDSO issue.
> >> >> >
> >> >> > But the VVAR macro will be not a part of this patch set. If you depend
> >> >> > on this, feel free to create one. From my point of view this is not
> >> >> > feasible without a macro hacking, because the address accessing the vvar
> >> >> > area differs in kernel and VDSO user mode.
> >> >>
> >> >> Sorry, but "I will make the code messier for no apparent reason and I
> >> >> will not offer to fix it in the same series" gets my NAK.
> >> >>
> >> >> Hint: I'm talking about two or three lines of code in vvar.h.
> >> >>
> >> >
> >> > A hint back: if you threat me with a NAK for a requested code sequence
> >> > which currently no user, this is far away from professional. I am not
> >> > your trainee.
> >> >
> >> > BTW: If it is so easy, send me the two or three lines and i will merge
> >> > it ;-)
> >>
> >> Something to the effect of:
> >>
> >> #elif defined(BUILD_VDSO32)
> >> #define VVAR(name) (*vvar_ ## name)
> >> #endif
> >>
> >> Should do the trick.
> >
> > You are wrong...
> >
> > #ifdef BUILD_VDSO32
> >
> > #define DECLARE_VVAR(offset, type, name) \
> >         extern type vvar_ ## name __attribute__((visibility("hidden")));
> >
> > #define VVAR(name) (vvar_ ## name)
> >
> > #else
> >
> > /* Base address of vvars.  This is not ABI. */
> > #ifdef CONFIG_X86_64
> > #define VVAR_ADDRESS (-10*1024*1024 - 4096)
> > #else
> > extern char __vvar_page;
> >
> > #define VVAR_ADDRESS (&__vvar_page)
> > #endif
> >
> > This would do the trick!
> >
> > But for 64 bit to 32 bit conversation layer in vclock_gettime.c there is
> > still a
> >
> > struct arch_vsyscall_gtod_data arch_vvar_vsyscall_gtod_data
> >         __attribute__((visibility("hidden")));
> > #define gtod (&arch_vvar_vsyscall_gtod_data)
> >
> > needed, because vvar_vsyscall_gtod_data is the 32 bit version, which
> > would result in incorrect access of the struct members. So my code can't
> > use this VVAR macro.
> 
> For 32-on-64, I must have read your code wrong.  Are you sticking two
> copies of the same struct with different layout into the vvar page?
> If so, wouldn't it be better to only have the variant with a common
> layout and use it for all versions of the vdso?
> 

No, only one. But for depend on 32/64 bit the layout differs.

We discuss this topic some days before, so please have a look at the
code and the previous posts.

- Stefani


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