Re: [PATCH 6/6] Add 32 bit VDSO support for 32 and 64 bit kernels

[Stefani Seibold]

Am Dienstag, den 18.12.2012, 12:37 -0800 schrieb Andy Lutomirski:
> On Tue, Dec 18, 2012 at 12:32 PM, Stefani Seibold <stefani@xxxxxxxxxxx> wrote:
> > Am Dienstag, den 18.12.2012, 10:44 -0800 schrieb H. Peter Anvin:
> >> On 12/18/2012 08:52 AM, Stefani Seibold wrote:
> >> >
> >> > Pardon, i never disregarded nor i have agreed that this is going to be a
> >> > part of the VDSO. I currently have also no idea how to do this and i see
> >> > no need at the moment to do this revamp. The 64 bit VDSO lives since
> >> > more than 6 years with this kind of implementation.
> >> >
> >>
> >> It was part of this discussion thread, about how to best manage the
> >> address space.  Fixed addresses are a major problem, and introducing new
> >> ones are extremely undesirable.
> >>
> >
> > There is no introduce of new fix address. There are still there for
> > x86_64. If this will currently not a major problem on this architecture
> > than it will not for x86_32 too.
> 
> Not necessarily true.  On x86-64 (non-compat) the fixmap address is in
> kernel space (high bit set), so it can't conflict with anything in
> user space.  On true 32-bit mode, the same applies.  In compat mode,
> the fixed address is in *user* space and might conflict with existing
> uses.
> 
> >
> >> Hence I wrote:
> >>
> >> > IMO it seems this is making it way more complicated than it is. Just
> >> > make sure you have a section in the vdso where you can map in a data
> >> > page with the symbols in the right offsets. Extra points for doing
> >> > magic so that it is at the beginning or end, but I think that might
> >> > be harder than necessary.
> >>
> >> Basically, make the vvar and hpet pages part of the vdso page list.
> >> Optionally they can be mapped without the MAYWRITE option -- in fact, we
> >> could easily split the vdso into an executable area which gets MAYWRITE
> >> to be able to set breakpoints and a data area which doesn't -- but that
> >> is a minor tweak IMO.
> >>
> >
> > I see the benefits, but it will not work under all circumstance. The
> > VDSO compat mode for x86_32 requires a fix address and there is no room
> > behind this. So since this must preserved, i see no real gain for this.
> 
> Not true.  It can be mapped with the vdso at a variable address using
> GOTOFF addressing.  See my earlier email with
> __attribute__((visibility("hidden")).
> 
> --Andy

I am not sure that we talking about the same.

In a 32 bit kernel a VDSO can mapped on an fix address VDSO_HIGH_BASE
(kernel parameter vdso32=2 or CONFIG_COMPAT_VDSO), which is 0xffffe000.
There is no available page before nor after this page, so there is no
space for a multi page VDSO.

The only way i see is to do this is a test in the vdso_...() functions
for running on this address and than access the old fixmap addresses for
VVAR and HPET. This can be done for example by a tweaking macro.

If this is okay, i can do it. Otherwise i have no idea how to.

- Stefani


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