Messages in this thread | | | Subject | Re: [PATCH 6/6] Add 32 bit VDSO support for 32 and 64 bit kernels | From | Stefani Seibold <> | Date | Thu, 20 Dec 2012 08:41:34 +0100 |
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Am Dienstag, den 18.12.2012, 12:37 -0800 schrieb Andy Lutomirski: > On Tue, Dec 18, 2012 at 12:32 PM, Stefani Seibold <stefani@seibold.net> 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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