On Fri, Sep 06, 2019 at 04:29:50PM -0500, Steve Wahl wrote: > Our hardware (UV aka Superdome Flex) has address ranges marked > reserved by the BIOS. These ranges can cause the system to halt if > accessed. > > During kernel initialization, the processor was speculating into > reserved memory causing system halts. The processor speculation is > enabled because the reserved memory is being mapped by the kernel. > > The page table level2_kernel_pgt is 1 GiB in size, and had all pages > initially marked as valid, and the kernel is placed anywhere in this > range depending on the virtual address selected by KASLR. Later on in > the boot process, the valid area gets trimmed back to the space > occupied by the kernel. > > But during the interval of time when the full 1 GiB space was marked > as valid, if the kernel physical address chosen by KASLR was close > enough to our reserved memory regions, the valid pages outside the > actual kernel space were allowing the processor to issue speculative > accesses to the reserved space, causing the system to halt. > > This was encountered somewhat rarely on a normal system boot, and > somewhat more often when starting the crash kernel if > "crashkernel=512M,high" was specified on the command line (because > this heavily restricts the physical address of the crash kernel, > usually to within 1 GiB of our reserved space). > > The answer is to invalidate the pages of this table outside the > address range occupied by the kernel before the page table is > activated. This patch has been validated to fix this problem on our > hardware.
If the goal is to avoid *any* mapping of the reserved region to stop speculation, I don't think this patch will do the job. We still (likely) have the same memory mapped as part of the identity mapping. And it happens at least in two places: here and before on decompression stage. -- Kirill A. Shutemov
