Messages in this thread | | | Date | Fri, 28 Aug 2015 09:02:51 -0700 | From | Dave Hansen <> | Subject | Re: [PATCH 11/11] x86, fpu: check CPU-provided sizes against struct declarations |
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On 08/27/2015 10:25 PM, Ingo Molnar wrote: > * Dave Hansen <dave@sr71.net> wrote: >> @@ -447,6 +492,14 @@ static void do_extra_xstate_size_checks( >> paranoid_xstate_size += xfeature_size(i); >> } >> XSTATE_WARN_ON(paranoid_xstate_size != xstate_size); >> + /* >> + * Basically, make sure that XSTATE_RESERVE has forced >> + * xregs_state to be large enough. This is not fatal >> + * because we reserve a *lot* of extra room in the init >> + * task struct, but we should at least know we got it >> + * wrong. >> + */ >> + XSTATE_WARN_ON(xstate_size > sizeof(struct xregs_state)); > > So do we need to warn about this? arch_task_struct_size is already dynamic today.
I'm unsure what _actually_ blew up, but I missed adding protection keys and AVX-512 to XSTATE_RESERVE and the kernel crashed the first time I did a non-init-state-PKRU XSAVE.
> The only problem would be the init task, which is allocated statically - can we > fix that?
We could theoretically make it dynamic, but I'm really not sure it's worth the trouble. I just removed the init_task=INIT_TASK() initialization to see what would happen and something blew up early (last I saw on the console was the "early console in setup code").
The current size of the non-XSAVE data in task_struct is ~2k. The xsave data is 800-something bytes, so say ~1k. Our init_task ends up being ~6k, 3k of which is wasted. On an AVX-512 CPU, that means 1k of waste.
From how early things died, I'm going to go out on a limb and say that we'll need to bootmem alloc our new dynamic init_task and probably can't practically wait for the slab to show up. Bootmem can only do full pages, so our 6k can be trimmed to 4k. On an AVX-512 CPU, the 6k goes *up* to 8k.
It doesn't look like a fun exercise for 2k of memory savings.
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