Messages in this thread | | | Subject | Re: [PATCH 6/6] Add 32 bit VDSO support for 32 and 64 bit kernels | From | "H. Peter Anvin" <> | Date | Thu, 20 Dec 2012 08:17:41 -0800 |
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The whole point is to avoid fixed addresses. We already install a list of pages as the vdso; the vvar and hpet pages can simply be part of that list.
Stefani Seibold <stefani@seibold.net> wrote:
>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
-- Sent from my mobile phone. Please excuse brevity and lack of formatting.
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