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    SubjectRe: 4.17.x won't boot due to "x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G"
    On Thu, Jul 26, 2018 at 04:21:11PM +0000, Dmitry Malkin wrote:
    > On 07/26/2018 04:50 PM, Kirill A. Shutemov wrote:
    > > > > > 2. reading from memory which may be reserved in case of EFI systems:
    > > > > > > >    ebda_start = *(unsigned short *)0x40e << 4;
    > > > > > > >    bios_start = *(unsigned short *)0x413 << 10;
    > > > > > Also, on EFI system without CSM it will results in all zeros. Which will
    > > > > > place trampoline_start to 0x9d000. And it also may be reserved memory. In
    > > > > > fact I have such system and it is causes instant reboot (when code starts
    > > > > > copying to "trampoline_start").
    > > > > Could you show dmesg from such system?
    > > > Sure, here it is (please note than not both pages are reserved but only
    > > > second one: 0x9e000-0x9ffff):
    > > Well. That's bad.
    > >
    > > I don't see much options but parse e820 in decompression code. I hoped to
    > > avoid this.
    > >
    > > Let me see what I can do there.
    > Just in case of UEFI (I don't know much about BIOS and kexec):
    > register RSI (right before call paging_prepare) will contains pointer to
    > "struct boot_params" (returned by function efi_main() in eboot.c).
    > There are fields e820_table and e820_entries.
    >

    Could you check if this makes a difference for you?

    diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
    index 8c5107545251..9e2157371491 100644
    --- a/arch/x86/boot/compressed/pgtable_64.c
    +++ b/arch/x86/boot/compressed/pgtable_64.c
    @@ -1,3 +1,4 @@
    +#include <asm/e820/types.h>
    #include <asm/processor.h>
    #include "pgtable.h"
    #include "../string.h"
    @@ -34,10 +35,62 @@ unsigned long *trampoline_32bit __section(.data);
    extern struct boot_params *boot_params;
    int cmdline_find_option_bool(const char *option);

    +static unsigned long find_trampoline_placement(void)
    +{
    + unsigned long bios_start, ebda_start;
    + unsigned long trampoline_start;
    + struct boot_e820_entry *entry;
    + int i;
    +
    + /*
    + * Find a suitable spot for the trampoline.
    + * This code is based on reserve_bios_regions().
    + */
    +
    + ebda_start = *(unsigned short *)0x40e << 4;
    + bios_start = *(unsigned short *)0x413 << 10;
    +
    + if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
    + bios_start = BIOS_START_MAX;
    +
    + if (ebda_start > BIOS_START_MIN && ebda_start < bios_start)
    + bios_start = ebda_start;
    +
    + bios_start = round_down(bios_start, PAGE_SIZE);
    +
    + /* Find the first usable memory region under bios_start. */
    + for (i = boot_params->e820_entries - 1; i >= 0; i--) {
    + entry = &boot_params->e820_table[i];
    +
    + /* Skip all entries above bios_start. */
    + if (bios_start <= entry->addr)
    + continue;
    +
    + /* Skip non-RAM entries. */
    + if (entry->type != E820_TYPE_RAM)
    + continue;
    +
    + /* Adjust bios_start to the end of the entry if needed. */
    + if (bios_start > entry->addr + entry->size)
    + bios_start = entry->addr + entry->size;
    +
    + /* Keep bios_start page-aligned. */
    + bios_start = round_down(bios_start, PAGE_SIZE);
    +
    + /* Skip the entry if it's too small. */
    + if (bios_start - TRAMPOLINE_32BIT_SIZE < entry->addr)
    + continue;
    +
    + break;
    + }
    +
    + /* Place the trampoline just below the end of low memory */
    + return bios_start - TRAMPOLINE_32BIT_SIZE;
    +}
    +
    struct paging_config paging_prepare(void *rmode)
    {
    struct paging_config paging_config = {};
    - unsigned long bios_start, ebda_start;

    /* Initialize boot_params. Required for cmdline_find_option_bool(). */
    boot_params = rmode;
    @@ -61,23 +114,7 @@ struct paging_config paging_prepare(void *rmode)
    paging_config.l5_required = 1;
    }

    - /*
    - * Find a suitable spot for the trampoline.
    - * This code is based on reserve_bios_regions().
    - */
    -
    - ebda_start = *(unsigned short *)0x40e << 4;
    - bios_start = *(unsigned short *)0x413 << 10;
    -
    - if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
    - bios_start = BIOS_START_MAX;
    -
    - if (ebda_start > BIOS_START_MIN && ebda_start < bios_start)
    - bios_start = ebda_start;
    -
    - /* Place the trampoline just below the end of low memory, aligned to 4k */
    - paging_config.trampoline_start = bios_start - TRAMPOLINE_32BIT_SIZE;
    - paging_config.trampoline_start = round_down(paging_config.trampoline_start, PAGE_SIZE);
    + paging_config.trampoline_start = find_trampoline_placement();

    trampoline_32bit = (unsigned long *)paging_config.trampoline_start;

    --
    Kirill A. Shutemov
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