Andrea Arcangeli writes:
> Speed is not an issue, cr4 would never be tweaked unless you use
> seccomp, the cr4 flip is in an extreme slow path.
>
> I think there are two ways to solve this race:
>
> 1) why don't we read the cr4 from the cpu? would movl %%cr4, %%eax
> generate a general protection fault? Can the cr4 be read in ring 0?
> Why to read it from memory if we've that information in the cpu already?
Yes the kernel can read cr4. I believe the best solution is to modify
__flush_tlb_global() to read cr4's current value and mix it in when
toggling CR4.PGE and uploading mmu_cr4_features.
(But please make it conditional on CONFIG_SECCOMP.)
The code called from __switch_to() would have to set or clear cr4
locally only. That's easy using write_cr4() and read_cr4().
/Mikael
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