From: Ard Biesheuvel <ardb@kernel.org>

The startup code that constructs the kernel virtual mapping runs from
the 1:1 mapping of memory itself, and therefore, cannot use absolute
symbol references. Move this code into a separate source file under
arch/x86/boot/startup/ where all such code will be kept from now on.

Since all code here is constructed in a manner that ensures that it
tolerates running from the 1:1 mapping of memory, any uses of the
RIP_REL_REF() macro can be dropped, along with __head annotations for
placing this code in a dedicated startup section.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/startup/Makefile     |   2 +-
 arch/x86/boot/startup/map_kernel.c | 215 ++++++++++++++++++++
 arch/x86/kernel/head64.c           | 211 +------------------
 3 files changed, 217 insertions(+), 211 deletions(-)

diff --git a/arch/x86/boot/startup/Makefile b/arch/x86/boot/startup/Makefile
index 34b324cbd5a4..01423063fec2 100644
--- a/arch/x86/boot/startup/Makefile
+++ b/arch/x86/boot/startup/Makefile
@@ -15,7 +15,7 @@ KMSAN_SANITIZE	:= n
 UBSAN_SANITIZE	:= n
 KCOV_INSTRUMENT	:= n
 
-obj-$(CONFIG_X86_64)		+= gdt_idt.o
+obj-$(CONFIG_X86_64)		+= gdt_idt.o map_kernel.o
 
 lib-$(CONFIG_X86_64)		+= la57toggle.o
 lib-$(CONFIG_EFI_MIXED)		+= efi-mixed.o
diff --git a/arch/x86/boot/startup/map_kernel.c b/arch/x86/boot/startup/map_kernel.c
new file mode 100644
index 000000000000..1cf57c03c319
--- /dev/null
+++ b/arch/x86/boot/startup/map_kernel.c
@@ -0,0 +1,215 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/init.h>
+#include <linux/linkage.h>
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/pgtable.h>
+
+#include <asm/sections.h>
+#include <asm/setup.h>
+#include <asm/sev.h>
+
+extern pmd_t early_dynamic_pgts[EARLY_DYNAMIC_PAGE_TABLES][PTRS_PER_PMD];
+extern unsigned int next_early_pgt;
+
+static inline bool check_la57_support(void)
+{
+	if (!IS_ENABLED(CONFIG_X86_5LEVEL))
+		return false;
+
+	/*
+	 * 5-level paging is detected and enabled at kernel decompression
+	 * stage. Only check if it has been enabled there.
+	 */
+	if (!(native_read_cr4() & X86_CR4_LA57))
+		return false;
+
+	__pgtable_l5_enabled	= 1;
+	pgdir_shift		= 48;
+	ptrs_per_p4d		= 512;
+	page_offset_base	= __PAGE_OFFSET_BASE_L5;
+	vmalloc_base		= __VMALLOC_BASE_L5;
+	vmemmap_base		= __VMEMMAP_BASE_L5;
+
+	return true;
+}
+
+static unsigned long sme_postprocess_startup(struct boot_params *bp,
+					     pmdval_t *pmd,
+					     unsigned long p2v_offset)
+{
+	unsigned long paddr, paddr_end;
+	int i;
+
+	/* Encrypt the kernel and related (if SME is active) */
+	sme_encrypt_kernel(bp);
+
+	/*
+	 * Clear the memory encryption mask from the .bss..decrypted section.
+	 * The bss section will be memset to zero later in the initialization so
+	 * there is no need to zero it after changing the memory encryption
+	 * attribute.
+	 */
+	if (sme_get_me_mask()) {
+		paddr = (unsigned long)__start_bss_decrypted;
+		paddr_end = (unsigned long)__end_bss_decrypted;
+
+		for (; paddr < paddr_end; paddr += PMD_SIZE) {
+			/*
+			 * On SNP, transition the page to shared in the RMP table so that
+			 * it is consistent with the page table attribute change.
+			 *
+			 * __start_bss_decrypted has a virtual address in the high range
+			 * mapping (kernel .text). PVALIDATE, by way of
+			 * early_snp_set_memory_shared(), requires a valid virtual
+			 * address but the kernel is currently running off of the identity
+			 * mapping so use the PA to get a *currently* valid virtual address.
+			 */
+			early_snp_set_memory_shared(paddr, paddr, PTRS_PER_PMD);
+
+			i = pmd_index(paddr - p2v_offset);
+			pmd[i] -= sme_get_me_mask();
+		}
+	}
+
+	/*
+	 * Return the SME encryption mask (if SME is active) to be used as a
+	 * modifier for the initial pgdir entry programmed into CR3.
+	 */
+	return sme_get_me_mask();
+}
+
+unsigned long __init __startup_64(unsigned long p2v_offset,
+				  struct boot_params *bp)
+{
+	pmd_t (*early_pgts)[PTRS_PER_PMD] = early_dynamic_pgts;
+	unsigned long physaddr = (unsigned long)_text;
+	unsigned long va_text, va_end;
+	unsigned long pgtable_flags;
+	unsigned long load_delta;
+	pgdval_t *pgd;
+	p4dval_t *p4d;
+	pudval_t *pud;
+	pmdval_t *pmd, pmd_entry;
+	bool la57;
+	int i;
+
+	la57 = check_la57_support();
+
+	/* Is the address too large? */
+	if (physaddr >> MAX_PHYSMEM_BITS)
+		for (;;);
+
+	/*
+	 * Compute the delta between the address I am compiled to run at
+	 * and the address I am actually running at.
+	 */
+	phys_base = load_delta = __START_KERNEL_map + p2v_offset;
+
+	/* Is the address not 2M aligned? */
+	if (load_delta & ~PMD_MASK)
+		for (;;);
+
+	va_text = physaddr - p2v_offset;
+	va_end  = (unsigned long)_end - p2v_offset;
+
+	/* Include the SME encryption mask in the fixup value */
+	load_delta += sme_get_me_mask();
+
+	/* Fixup the physical addresses in the page table */
+
+	pgd = &early_top_pgt[0].pgd;
+	pgd[pgd_index(__START_KERNEL_map)] += load_delta;
+
+	if (IS_ENABLED(CONFIG_X86_5LEVEL) && la57) {
+		p4d = (p4dval_t *)level4_kernel_pgt;
+		p4d[MAX_PTRS_PER_P4D - 1] += load_delta;
+
+		pgd[pgd_index(__START_KERNEL_map)] = (pgdval_t)p4d | _PAGE_TABLE;
+	}
+
+	level3_kernel_pgt[PTRS_PER_PUD - 2].pud += load_delta;
+	level3_kernel_pgt[PTRS_PER_PUD - 1].pud += load_delta;
+
+	for (i = FIXMAP_PMD_TOP; i > FIXMAP_PMD_TOP - FIXMAP_PMD_NUM; i--)
+		level2_fixmap_pgt[i].pmd += load_delta;
+
+	/*
+	 * Set up the identity mapping for the switchover.  These
+	 * entries should *NOT* have the global bit set!  This also
+	 * creates a bunch of nonsense entries but that is fine --
+	 * it avoids problems around wraparound.
+	 */
+
+	pud = &early_pgts[0]->pmd;
+	pmd = &early_pgts[1]->pmd;
+	next_early_pgt = 2;
+
+	pgtable_flags = _KERNPG_TABLE_NOENC + sme_get_me_mask();
+
+	if (la57) {
+		p4d = &early_pgts[next_early_pgt++]->pmd;
+
+		i = (physaddr >> PGDIR_SHIFT) % PTRS_PER_PGD;
+		pgd[i + 0] = (pgdval_t)p4d + pgtable_flags;
+		pgd[i + 1] = (pgdval_t)p4d + pgtable_flags;
+
+		i = physaddr >> P4D_SHIFT;
+		p4d[(i + 0) % PTRS_PER_P4D] = (pgdval_t)pud + pgtable_flags;
+		p4d[(i + 1) % PTRS_PER_P4D] = (pgdval_t)pud + pgtable_flags;
+	} else {
+		i = (physaddr >> PGDIR_SHIFT) % PTRS_PER_PGD;
+		pgd[i + 0] = (pgdval_t)pud + pgtable_flags;
+		pgd[i + 1] = (pgdval_t)pud + pgtable_flags;
+	}
+
+	i = physaddr >> PUD_SHIFT;
+	pud[(i + 0) % PTRS_PER_PUD] = (pudval_t)pmd + pgtable_flags;
+	pud[(i + 1) % PTRS_PER_PUD] = (pudval_t)pmd + pgtable_flags;
+
+	pmd_entry = __PAGE_KERNEL_LARGE_EXEC & ~_PAGE_GLOBAL;
+	/* Filter out unsupported __PAGE_KERNEL_* bits: */
+	pmd_entry &= __supported_pte_mask;
+	pmd_entry += sme_get_me_mask();
+	pmd_entry +=  physaddr;
+
+	for (i = 0; i < DIV_ROUND_UP(va_end - va_text, PMD_SIZE); i++) {
+		int idx = i + (physaddr >> PMD_SHIFT);
+
+		pmd[idx % PTRS_PER_PMD] = pmd_entry + i * PMD_SIZE;
+	}
+
+	/*
+	 * Fixup the kernel text+data virtual addresses. Note that
+	 * we might write invalid pmds, when the kernel is relocated
+	 * cleanup_highmap() fixes this up along with the mappings
+	 * beyond _end.
+	 *
+	 * Only the region occupied by the kernel image has so far
+	 * been checked against the table of usable memory regions
+	 * provided by the firmware, so invalidate pages outside that
+	 * region. A page table entry that maps to a reserved area of
+	 * memory would allow processor speculation into that area,
+	 * and on some hardware (particularly the UV platform) even
+	 * speculative access to some reserved areas is caught as an
+	 * error, causing the BIOS to halt the system.
+	 */
+
+	pmd = &level2_kernel_pgt[0].pmd;
+
+	/* invalidate pages before the kernel image */
+	for (i = 0; i < pmd_index(va_text); i++)
+		pmd[i] &= ~_PAGE_PRESENT;
+
+	/* fixup pages that are part of the kernel image */
+	for (; i <= pmd_index(va_end); i++)
+		if (pmd[i] & _PAGE_PRESENT)
+			pmd[i] += load_delta;
+
+	/* invalidate pages after the kernel image */
+	for (; i < PTRS_PER_PMD; i++)
+		pmd[i] &= ~_PAGE_PRESENT;
+
+	return sme_postprocess_startup(bp, pmd, p2v_offset);
+}
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 5b993b545c7e..6b68a206fa7f 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -47,7 +47,7 @@
  * Manage page tables very early on.
  */
 extern pmd_t early_dynamic_pgts[EARLY_DYNAMIC_PAGE_TABLES][PTRS_PER_PMD];
-static unsigned int __initdata next_early_pgt;
+unsigned int __initdata next_early_pgt;
 pmdval_t early_pmd_flags = __PAGE_KERNEL_LARGE & ~(_PAGE_GLOBAL | _PAGE_NX);
 
 #ifdef CONFIG_X86_5LEVEL
@@ -67,215 +67,6 @@ unsigned long vmemmap_base __ro_after_init = __VMEMMAP_BASE_L4;
 EXPORT_SYMBOL(vmemmap_base);
 #endif
 
-static inline bool check_la57_support(void)
-{
-	if (!IS_ENABLED(CONFIG_X86_5LEVEL))
-		return false;
-
-	/*
-	 * 5-level paging is detected and enabled at kernel decompression
-	 * stage. Only check if it has been enabled there.
-	 */
-	if (!(native_read_cr4() & X86_CR4_LA57))
-		return false;
-
-	RIP_REL_REF(__pgtable_l5_enabled)	= 1;
-	RIP_REL_REF(pgdir_shift)		= 48;
-	RIP_REL_REF(ptrs_per_p4d)		= 512;
-	RIP_REL_REF(page_offset_base)		= __PAGE_OFFSET_BASE_L5;
-	RIP_REL_REF(vmalloc_base)		= __VMALLOC_BASE_L5;
-	RIP_REL_REF(vmemmap_base)		= __VMEMMAP_BASE_L5;
-
-	return true;
-}
-
-static unsigned long __head sme_postprocess_startup(struct boot_params *bp,
-						    pmdval_t *pmd,
-						    unsigned long p2v_offset)
-{
-	unsigned long paddr, paddr_end;
-	int i;
-
-	/* Encrypt the kernel and related (if SME is active) */
-	sme_encrypt_kernel(bp);
-
-	/*
-	 * Clear the memory encryption mask from the .bss..decrypted section.
-	 * The bss section will be memset to zero later in the initialization so
-	 * there is no need to zero it after changing the memory encryption
-	 * attribute.
-	 */
-	if (sme_get_me_mask()) {
-		paddr = (unsigned long)&RIP_REL_REF(__start_bss_decrypted);
-		paddr_end = (unsigned long)&RIP_REL_REF(__end_bss_decrypted);
-
-		for (; paddr < paddr_end; paddr += PMD_SIZE) {
-			/*
-			 * On SNP, transition the page to shared in the RMP table so that
-			 * it is consistent with the page table attribute change.
-			 *
-			 * __start_bss_decrypted has a virtual address in the high range
-			 * mapping (kernel .text). PVALIDATE, by way of
-			 * early_snp_set_memory_shared(), requires a valid virtual
-			 * address but the kernel is currently running off of the identity
-			 * mapping so use the PA to get a *currently* valid virtual address.
-			 */
-			early_snp_set_memory_shared(paddr, paddr, PTRS_PER_PMD);
-
-			i = pmd_index(paddr - p2v_offset);
-			pmd[i] -= sme_get_me_mask();
-		}
-	}
-
-	/*
-	 * Return the SME encryption mask (if SME is active) to be used as a
-	 * modifier for the initial pgdir entry programmed into CR3.
-	 */
-	return sme_get_me_mask();
-}
-
-/* Code in __startup_64() can be relocated during execution, but the compiler
- * doesn't have to generate PC-relative relocations when accessing globals from
- * that function. Clang actually does not generate them, which leads to
- * boot-time crashes. To work around this problem, every global pointer must
- * be accessed using RIP_REL_REF(). Kernel virtual addresses can be determined
- * by subtracting p2v_offset from the RIP-relative address.
- */
-unsigned long __head __startup_64(unsigned long p2v_offset,
-				  struct boot_params *bp)
-{
-	pmd_t (*early_pgts)[PTRS_PER_PMD] = RIP_REL_REF(early_dynamic_pgts);
-	unsigned long physaddr = (unsigned long)&RIP_REL_REF(_text);
-	unsigned long va_text, va_end;
-	unsigned long pgtable_flags;
-	unsigned long load_delta;
-	pgdval_t *pgd;
-	p4dval_t *p4d;
-	pudval_t *pud;
-	pmdval_t *pmd, pmd_entry;
-	bool la57;
-	int i;
-
-	la57 = check_la57_support();
-
-	/* Is the address too large? */
-	if (physaddr >> MAX_PHYSMEM_BITS)
-		for (;;);
-
-	/*
-	 * Compute the delta between the address I am compiled to run at
-	 * and the address I am actually running at.
-	 */
-	load_delta = __START_KERNEL_map + p2v_offset;
-	RIP_REL_REF(phys_base) = load_delta;
-
-	/* Is the address not 2M aligned? */
-	if (load_delta & ~PMD_MASK)
-		for (;;);
-
-	va_text = physaddr - p2v_offset;
-	va_end  = (unsigned long)&RIP_REL_REF(_end) - p2v_offset;
-
-	/* Include the SME encryption mask in the fixup value */
-	load_delta += sme_get_me_mask();
-
-	/* Fixup the physical addresses in the page table */
-
-	pgd = &RIP_REL_REF(early_top_pgt)->pgd;
-	pgd[pgd_index(__START_KERNEL_map)] += load_delta;
-
-	if (IS_ENABLED(CONFIG_X86_5LEVEL) && la57) {
-		p4d = (p4dval_t *)&RIP_REL_REF(level4_kernel_pgt);
-		p4d[MAX_PTRS_PER_P4D - 1] += load_delta;
-
-		pgd[pgd_index(__START_KERNEL_map)] = (pgdval_t)p4d | _PAGE_TABLE;
-	}
-
-	RIP_REL_REF(level3_kernel_pgt)[PTRS_PER_PUD - 2].pud += load_delta;
-	RIP_REL_REF(level3_kernel_pgt)[PTRS_PER_PUD - 1].pud += load_delta;
-
-	for (i = FIXMAP_PMD_TOP; i > FIXMAP_PMD_TOP - FIXMAP_PMD_NUM; i--)
-		RIP_REL_REF(level2_fixmap_pgt)[i].pmd += load_delta;
-
-	/*
-	 * Set up the identity mapping for the switchover.  These
-	 * entries should *NOT* have the global bit set!  This also
-	 * creates a bunch of nonsense entries but that is fine --
-	 * it avoids problems around wraparound.
-	 */
-
-	pud = &early_pgts[0]->pmd;
-	pmd = &early_pgts[1]->pmd;
-	RIP_REL_REF(next_early_pgt) = 2;
-
-	pgtable_flags = _KERNPG_TABLE_NOENC + sme_get_me_mask();
-
-	if (la57) {
-		p4d = &early_pgts[RIP_REL_REF(next_early_pgt)++]->pmd;
-
-		i = (physaddr >> PGDIR_SHIFT) % PTRS_PER_PGD;
-		pgd[i + 0] = (pgdval_t)p4d + pgtable_flags;
-		pgd[i + 1] = (pgdval_t)p4d + pgtable_flags;
-
-		i = physaddr >> P4D_SHIFT;
-		p4d[(i + 0) % PTRS_PER_P4D] = (pgdval_t)pud + pgtable_flags;
-		p4d[(i + 1) % PTRS_PER_P4D] = (pgdval_t)pud + pgtable_flags;
-	} else {
-		i = (physaddr >> PGDIR_SHIFT) % PTRS_PER_PGD;
-		pgd[i + 0] = (pgdval_t)pud + pgtable_flags;
-		pgd[i + 1] = (pgdval_t)pud + pgtable_flags;
-	}
-
-	i = physaddr >> PUD_SHIFT;
-	pud[(i + 0) % PTRS_PER_PUD] = (pudval_t)pmd + pgtable_flags;
-	pud[(i + 1) % PTRS_PER_PUD] = (pudval_t)pmd + pgtable_flags;
-
-	pmd_entry = __PAGE_KERNEL_LARGE_EXEC & ~_PAGE_GLOBAL;
-	/* Filter out unsupported __PAGE_KERNEL_* bits: */
-	pmd_entry &= RIP_REL_REF(__supported_pte_mask);
-	pmd_entry += sme_get_me_mask();
-	pmd_entry +=  physaddr;
-
-	for (i = 0; i < DIV_ROUND_UP(va_end - va_text, PMD_SIZE); i++) {
-		int idx = i + (physaddr >> PMD_SHIFT);
-
-		pmd[idx % PTRS_PER_PMD] = pmd_entry + i * PMD_SIZE;
-	}
-
-	/*
-	 * Fixup the kernel text+data virtual addresses. Note that
-	 * we might write invalid pmds, when the kernel is relocated
-	 * cleanup_highmap() fixes this up along with the mappings
-	 * beyond _end.
-	 *
-	 * Only the region occupied by the kernel image has so far
-	 * been checked against the table of usable memory regions
-	 * provided by the firmware, so invalidate pages outside that
-	 * region. A page table entry that maps to a reserved area of
-	 * memory would allow processor speculation into that area,
-	 * and on some hardware (particularly the UV platform) even
-	 * speculative access to some reserved areas is caught as an
-	 * error, causing the BIOS to halt the system.
-	 */
-
-	pmd = &RIP_REL_REF(level2_kernel_pgt)->pmd;
-
-	/* invalidate pages before the kernel image */
-	for (i = 0; i < pmd_index(va_text); i++)
-		pmd[i] &= ~_PAGE_PRESENT;
-
-	/* fixup pages that are part of the kernel image */
-	for (; i <= pmd_index(va_end); i++)
-		if (pmd[i] & _PAGE_PRESENT)
-			pmd[i] += load_delta;
-
-	/* invalidate pages after the kernel image */
-	for (; i < PTRS_PER_PMD; i++)
-		pmd[i] &= ~_PAGE_PRESENT;
-
-	return sme_postprocess_startup(bp, pmd, p2v_offset);
-}
-
 /* Wipe all early page tables except for the kernel symbol map */
 static void __init reset_early_page_tables(void)
 {
-- 
2.49.0.504.g3bcea36a83-goog

* Ard Biesheuvel <ardb+git@google.com> wrote:

> From: Ard Biesheuvel <ardb@kernel.org>
> 
> The startup code that constructs the kernel virtual mapping runs from 
> the 1:1 mapping of memory itself, and therefore, cannot use absolute 
> symbol references. Move this code into a separate source file under 
> arch/x86/boot/startup/ where all such code will be kept from now on.
> 
> Since all code here is constructed in a manner that ensures that it 
> tolerates running from the 1:1 mapping of memory, any uses of the 
> RIP_REL_REF() macro can be dropped, along with __head annotations for 
> placing this code in a dedicated startup section.

So would it be possible to do this in ~3 steps: first the mechanic 
movement of code, with very few changes (if the result builds & boots), 
then drop the RIP_REL_REF() uses and __head annotations in two separate 
patches?

Bisectability, ease of review, etc.

(The tiny bird gets the worm, but I might have butchered that proverb.)

Thanks,

	Ingo

On Mon, 7 Apr 2025 at 19:44, Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Ard Biesheuvel <ardb+git@google.com> wrote:
>
> > From: Ard Biesheuvel <ardb@kernel.org>
> >
> > The startup code that constructs the kernel virtual mapping runs from
> > the 1:1 mapping of memory itself, and therefore, cannot use absolute
> > symbol references. Move this code into a separate source file under
> > arch/x86/boot/startup/ where all such code will be kept from now on.
> >
> > Since all code here is constructed in a manner that ensures that it
> > tolerates running from the 1:1 mapping of memory, any uses of the
> > RIP_REL_REF() macro can be dropped, along with __head annotations for
> > placing this code in a dedicated startup section.
>
> So would it be possible to do this in ~3 steps: first the mechanic
> movement of code, with very few changes (if the result builds & boots),
> then drop the RIP_REL_REF() uses and __head annotations in two separate
> patches?
>
> Bisectability, ease of review, etc.
>
> (The tiny bird gets the worm, but I might have butchered that proverb.)
>

Yes.

And actually, the Clang boot regression that was reported indicates
that this statement it not 100% true to begin with. While it is no
longer necessary to use RIP_REL_REF() for accesses to global
variables, it may still be needed when explicitly taking the address
of a global variable and storing it in a stack allocated struct, e.g.,

void __init startup_64_setup_gdt_idt(void)
{
        void *handler = NULL;

        struct desc_ptr startup_gdt_descr = {
                .address = (__force unsigned long)gdt_page.gdt,
                .size    = GDT_SIZE - 1,
        };

In this case, even -fPIC may produce an absolute reference to
gdt_page.gdt, but from .rodata not from .text, and this is equally
broken at early boot.

Once all this code has been moved into place, I'll propose the
validation (similar to arm64 and EFI stub) which just greps the output
of readelf -r and checks for occurrences of R_X86_64_64; that way, we
will detect early and precisely whether the codegen is ok.

Please let me know which of these patches you are intending to keep in
tip/x86/boot, and I will respin on top of that.

* Ard Biesheuvel <ardb@kernel.org> wrote:

> On Mon, 7 Apr 2025 at 19:44, Ingo Molnar <mingo@kernel.org> wrote:
> >
> >
> > * Ard Biesheuvel <ardb+git@google.com> wrote:
> >
> > > From: Ard Biesheuvel <ardb@kernel.org>
> > >
> > > The startup code that constructs the kernel virtual mapping runs from
> > > the 1:1 mapping of memory itself, and therefore, cannot use absolute
> > > symbol references. Move this code into a separate source file under
> > > arch/x86/boot/startup/ where all such code will be kept from now on.
> > >
> > > Since all code here is constructed in a manner that ensures that it
> > > tolerates running from the 1:1 mapping of memory, any uses of the
> > > RIP_REL_REF() macro can be dropped, along with __head annotations for
> > > placing this code in a dedicated startup section.
> >
> > So would it be possible to do this in ~3 steps: first the mechanic
> > movement of code, with very few changes (if the result builds & boots),
> > then drop the RIP_REL_REF() uses and __head annotations in two separate
> > patches?
> >
> > Bisectability, ease of review, etc.
> >
> > (The tiny bird gets the worm, but I might have butchered that proverb.)
> >
> 
> Yes.
> 
> And actually, the Clang boot regression that was reported indicates 
> that this statement it not 100% true to begin with. While it is no 
> longer necessary to use RIP_REL_REF() for accesses to global 
> variables, it may still be needed when explicitly taking the address 
> of a global variable and storing it in a stack allocated struct, 
> e.g.,
> 
> void __init startup_64_setup_gdt_idt(void)
> {
>         void *handler = NULL;
> 
>         struct desc_ptr startup_gdt_descr = {
>                 .address = (__force unsigned long)gdt_page.gdt,
>                 .size    = GDT_SIZE - 1,
>         };
> 
> In this case, even -fPIC may produce an absolute reference to
> gdt_page.gdt, but from .rodata not from .text, and this is equally
> broken at early boot.

OK.

> Once all this code has been moved into place, I'll propose the
> validation (similar to arm64 and EFI stub) which just greps the output
> of readelf -r and checks for occurrences of R_X86_64_64; that way, we
> will detect early and precisely whether the codegen is ok.

Yeah, that sounds good!

> Please let me know which of these patches you are intending to keep 
> in tip/x86/boot, and I will respin on top of that.

I'd like to merge all of them as long as they don't intentionally regress.
All of this seemed like a step forward, and having them in one place
will enable a new type of debugging check - which is a win too in my book.

So no fundamental worries from me, just the request to have more 
careful iterations.

Thanks,

	Ingo