Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 18 Dec 2012 12:37:10 -0800 | Subject | Re: [PATCH 6/6] Add 32 bit VDSO support for 32 and 64 bit kernels |
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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
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