Re: [Xen-devel] [PATCH v3 2/5] x86/boot: Split bootsym() into four types of relocations

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.08.2019 18:35, David Woodhouse wrote:
> --- a/xen/arch/x86/boot/head.S
> +++ b/xen/arch/x86/boot/head.S
> @@ -699,14 +699,30 @@ trampoline_setup:
>          cmp     $sym_offs(__trampoline_rel_stop),%edi
>          jb      1b
>  
> -        /* Patch in the trampoline segment. */
> +        mov     $sym_offs(__trampoline32_rel_start),%edi
> +1:
> +        mov     %fs:(%edi),%eax
> +        add     %edx,%fs:(%edi,%eax)
> +        add     $4,%edi
> +        cmp     $sym_offs(__trampoline32_rel_stop),%edi
> +        jb      1b
> +
> +        mov     $sym_offs(__bootsym_rel_start),%edi
> +1:
> +        mov     %fs:(%edi),%eax
> +        add     %edx,%fs:(%edi,%eax)
> +        add     $4,%edi
> +        cmp     $sym_offs(__bootsym_rel_stop),%edi
> +        jb      1b

With the smaller sets now - are we risking misbehavior if one
of the relocation sets ends up empty? This wasn't reasonable to
expect before, but I think it would be nice to have a build-time
check rather than a hard to debug crash in case this happens.

> --- a/xen/arch/x86/boot/trampoline.S
> +++ b/xen/arch/x86/boot/trampoline.S
> @@ -16,21 +16,62 @@
>   * not guaranteed to persist.
>   */
>  
> -/* NB. bootsym() is only usable in real mode, or via BOOT_PSEUDORM_DS. */
> +/*
> + * There are four sets of relocations:
> + *
> + * bootsym():     Boot-time code relocated to low memory and run only once.
> + *                Only usable at boot; in real mode or via BOOT_PSEUDORM_DS.
> + * bootdatasym(): Boot-time BIOS-discovered data, relocated back up to Xen
> + *                image after discovery.
> + * trampsym():    Permanent trampoline code relocated into low memory for AP
> + *                startup and wakeup.
> + * tramp32sym():  32-bit trampoline code which at boot can be used directly
> + *                from the Xen image in memory, but which will need to be
> + *                relocated into low (well, into *mapped*) memory in order
> + *                to be used for AP startup.
> + */
>  #undef bootsym
>  #define bootsym(s) ((s)-trampoline_start)
>  
>  #define bootsym_rel(sym, off, opnd...)     \
>          bootsym(sym),##opnd;               \
>  111:;                                      \
> -        .pushsection .trampoline_rel, "a"; \
> +        .pushsection .bootsym_rel, "a";    \
>          .long 111b - (off) - .;            \
>          .popsection
>  
>  #define bootsym_segrel(sym, off)           \
>          $0,$bootsym(sym);                  \
>  111:;                                      \
> -        .pushsection .trampoline_seg, "a"; \
> +        .pushsection .bootsym_seg, "a";    \
> +        .long 111b - (off) - .;            \
> +        .popsection
> +
> +#define bootdatasym(s) ((s)-trampoline_start)
> +#define bootdatasym_rel(sym, off, opnd...) \
> +        bootdatasym(sym),##opnd;           \
> +111:;                                      \
> +        .pushsection .bootdatasym_rel, "a";\
> +        .long 111b - (off) - .;            \
> +        .popsection
> +
> +#undef trampsym

Why this and ...

> +#define trampsym(s) ((s)-trampoline_start)
> +
> +#define trampsym_rel(sym, off, opnd...)    \
> +        trampsym(sym),##opnd;              \
> +111:;                                      \
> +        .pushsection .trampsym_rel, "a";   \
> +        .long 111b - (off) - .;            \
> +        .popsection
> +
> +#undef tramp32sym

... this #undef? You have none ahead of the bootdatasym #define-s,
and (other than for bootsym) there's not conflicting C level one
afaics.

> +#define tramp32sym(s) ((s)-trampoline_start)
> +
> +#define tramp32sym_rel(sym, off, opnd...)  \
> +        tramp32sym(sym),##opnd;            \
> +111:;                                      \
> +        .pushsection .tramp32sym_rel, "a"; \
>          .long 111b - (off) - .;            \
>          .popsection

After your reply to my comment regarding the redundancy here I've
checked (in your git branch) how things end up. Am I mistaken, or
are the trampsym and tramp32sym #define-s entirely identical
(except for the relocations section name)? Even between the others
there's little enough difference, so it continues to be unclear to
me why you think it's better to have four instances of about the
same (not entirely trivial) thing.

> @@ -48,16 +89,19 @@
>  GLOBAL(trampoline_realmode_entry)
>          mov     %cs,%ax
>          mov     %ax,%ds
> -        movb    $0xA5,bootsym(trampoline_cpu_started)
> +        movb    $0xA5,trampsym(trampoline_cpu_started)
>          cld
>          cli
> -        lidt    bootsym(idt_48)
> -        lgdt    bootsym(gdt_48)
> +        lidt    trampsym(idt_48)
> +        lgdt    trampsym(gdt_48)
>          mov     $1,%bl                    # EBX != 0 indicates we are an AP
>          xor     %ax, %ax
>          inc     %ax
>          lmsw    %ax                       # CR0.PE = 1 (enter protected mode)
> -        ljmpl   $BOOT_CS32,$bootsym_rel(trampoline_protmode_entry,6)
> +        ljmpl   $BOOT_CS32,$tramp32sym_rel(trampoline_protmode_entry,6)
> +
> +GLOBAL(trampoline_cpu_started)
> +        .byte   0

The movement of this item here seems unrelated to this change; it's
also not mentioned in the description.

> @@ -115,10 +115,10 @@ static void __init relocate_trampoline(unsigned long 
> phys)
>            trampoline_ptr < __trampoline_rel_stop;
>            ++trampoline_ptr )
>          *(u32 *)(*trampoline_ptr + (long)trampoline_ptr) += phys;
> -    for ( trampoline_ptr = __trampoline_seg_start;
> -          trampoline_ptr < __trampoline_seg_stop;
> +    for ( trampoline_ptr = __trampoline32_rel_start;
> +          trampoline_ptr < __trampoline32_rel_stop;
>            ++trampoline_ptr )
> -        *(u16 *)(*trampoline_ptr + (long)trampoline_ptr) = phys >> 4;
> +        *(u32 *)(*trampoline_ptr + (long)trampoline_ptr) += phys;
>  }

Seeing this and adding in the comment about the redundant tramp*sym
macros I wonder why the relocations can't be put together in a single
section, and there be just a single loop here. (I realize this
entire function gets deleted from here later on, but anyway.)

Jan

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