On 08/27/2015 10:25 PM, Ingo Molnar wrote: > * Dave Hansen <[email protected]> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
