Re: [PATCH v4 11/11] x86/startup_64: Drop global variables keeping track of LA57 state
From: Brian Gerst
Date: Wed Feb 14 2024 - 10:25:14 EST
On Tue, Feb 13, 2024 at 7:42 AM Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
>
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> On x86_64, the core kernel is entered in long mode, which implies that
> paging is enabled. This means that the CR4.LA57 control bit is
> guaranteed to be in sync with the number of paging levels used by the
> kernel, and there is no need to store this in a variable.
>
> There is also no need to use variables for storing the calculations of
> pgdir_shift and ptrs_per_p4d, as they are easily determined on the fly.
>
> This removes the need for two different sources of truth for determining
> whether 5-level paging is in use: CR4.LA57 always reflects the actual
> state, and never changes from the point of view of the 64-bit core
> kernel. The only potential concern is the cost of CR4 accesses, which
> can be mitigated using alternatives patching based on feature detection.
>
> Note that even the decompressor does not manipulate any page tables
> before updating CR4.LA57, so it can also avoid the associated global
> variables entirely. However, as it does not implement alternatives
> patching, the associated ELF sections need to be discarded.
>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
> arch/x86/boot/compressed/misc.h | 4 --
> arch/x86/boot/compressed/pgtable_64.c | 12 ----
> arch/x86/boot/compressed/vmlinux.lds.S | 1 +
> arch/x86/include/asm/pgtable_64_types.h | 58 ++++++++------------
> arch/x86/kernel/cpu/common.c | 2 -
> arch/x86/kernel/head64.c | 33 +----------
> arch/x86/mm/kasan_init_64.c | 3 -
> arch/x86/mm/mem_encrypt_identity.c | 9 ---
> 8 files changed, 27 insertions(+), 95 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> index bc2f0f17fb90..2b15ddd0e177 100644
> --- a/arch/x86/boot/compressed/misc.h
> +++ b/arch/x86/boot/compressed/misc.h
> @@ -16,9 +16,6 @@
>
> #define __NO_FORTIFY
>
> -/* cpu_feature_enabled() cannot be used this early */
> -#define USE_EARLY_PGTABLE_L5
> -
> /*
> * Boot stub deals with identity mappings, physical and virtual addresses are
> * the same, so override these defines.
> @@ -178,7 +175,6 @@ static inline int count_immovable_mem_regions(void) { return 0; }
> #endif
>
> /* ident_map_64.c */
> -extern unsigned int __pgtable_l5_enabled, pgdir_shift, ptrs_per_p4d;
> extern void kernel_add_identity_map(unsigned long start, unsigned long end);
>
> /* Used by PAGE_KERN* macros: */
> diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
> index 51f957b24ba7..ae72f53f5e77 100644
> --- a/arch/x86/boot/compressed/pgtable_64.c
> +++ b/arch/x86/boot/compressed/pgtable_64.c
> @@ -9,13 +9,6 @@
> #define BIOS_START_MIN 0x20000U /* 128K, less than this is insane */
> #define BIOS_START_MAX 0x9f000U /* 640K, absolute maximum */
>
> -#ifdef CONFIG_X86_5LEVEL
> -/* __pgtable_l5_enabled needs to be in .data to avoid being cleared along with .bss */
> -unsigned int __section(".data") __pgtable_l5_enabled;
> -unsigned int __section(".data") pgdir_shift = 39;
> -unsigned int __section(".data") ptrs_per_p4d = 1;
> -#endif
> -
> /* Buffer to preserve trampoline memory */
> static char trampoline_save[TRAMPOLINE_32BIT_SIZE];
>
> @@ -125,11 +118,6 @@ asmlinkage void configure_5level_paging(struct boot_params *bp, void *pgtable)
> native_cpuid_eax(0) >= 7 &&
> (native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31)))) {
> l5_required = true;
> -
> - /* Initialize variables for 5-level paging */
> - __pgtable_l5_enabled = 1;
> - pgdir_shift = 48;
> - ptrs_per_p4d = 512;
> }
>
> /*
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 083ec6d7722a..06358bb067fe 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -81,6 +81,7 @@ SECTIONS
> *(.dynamic) *(.dynsym) *(.dynstr) *(.dynbss)
> *(.hash) *(.gnu.hash)
> *(.note.*)
> + *(.altinstructions .altinstr_replacement)
> }
>
> .got.plt (INFO) : {
> diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
> index 38b54b992f32..6a57bfdff52b 100644
> --- a/arch/x86/include/asm/pgtable_64_types.h
> +++ b/arch/x86/include/asm/pgtable_64_types.h
> @@ -6,7 +6,10 @@
>
> #ifndef __ASSEMBLY__
> #include <linux/types.h>
> +#include <asm/alternative.h>
> +#include <asm/cpufeatures.h>
> #include <asm/kaslr.h>
> +#include <asm/processor-flags.h>
>
> /*
> * These are used to make use of C type-checking..
> @@ -21,63 +24,50 @@ typedef unsigned long pgprotval_t;
> typedef struct { pteval_t pte; } pte_t;
> typedef struct { pmdval_t pmd; } pmd_t;
>
> -#ifdef CONFIG_X86_5LEVEL
> -extern unsigned int __pgtable_l5_enabled;
> -
> -#ifdef USE_EARLY_PGTABLE_L5
> -/*
> - * cpu_feature_enabled() is not available in early boot code.
> - * Use variable instead.
> - */
> -static inline bool pgtable_l5_enabled(void)
> +static __always_inline __pure bool pgtable_l5_enabled(void)
> {
> - return __pgtable_l5_enabled;
> -}
> -#else
> -#define pgtable_l5_enabled() cpu_feature_enabled(X86_FEATURE_LA57)
> -#endif /* USE_EARLY_PGTABLE_L5 */
> + unsigned long r;
> + bool ret;
>
> -#else
> -#define pgtable_l5_enabled() 0
> -#endif /* CONFIG_X86_5LEVEL */
> + if (!IS_ENABLED(CONFIG_X86_5LEVEL))
> + return false;
> +
> + asm(ALTERNATIVE_TERNARY(
> + "movq %%cr4, %[reg] \n\t btl %[la57], %k[reg]" CC_SET(c),
> + %P[feat], "stc", "clc")
> + : [reg] "=&r" (r), CC_OUT(c) (ret)
> + : [feat] "i" (X86_FEATURE_LA57),
> + [la57] "i" (X86_CR4_LA57_BIT)
> + : "cc");
This should be more like _static_cpu_has(), where the runtime test is
out of line in a discardable section, and the inline part is just a
JMP or NOP.
Brian Gerst