Re: [PATCH v4 11/11] x86/startup_64: Drop global variables keeping track of LA57 state

[Ard Biesheuvel]

On Wed, 14 Feb 2024 at 16:24, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>
> 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.
>

Why exactly? It matters very little in terms of space, a cross-section
jump is 5 bytes, and movq+btl is 7 bytes.

If you are referring to the use of the C flag: this way, it is left up
to the compiler to decide whether a branch or a conditional move is
more suitable, rather than forcing the use of a branch in one of the
two cases.