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Re: [PATCH RFC] x86/boot: Call efi_multiboot2() at it's linked address


  • To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Mon, 30 Sep 2024 14:49:31 +0200
  • Autocrypt: addr=jbeulich@xxxxxxxx; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL
  • Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Frediano Ziglio <frediano.ziglio@xxxxxxxxx>, Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>, Daniel Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Mon, 30 Sep 2024 12:49:52 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 30.09.2024 14:06, Andrew Cooper wrote:
> --- a/xen/arch/x86/boot/head.S
> +++ b/xen/arch/x86/boot/head.S
> @@ -344,6 +344,66 @@ __efi64_mb2_start:
>          lea     .Lmb2_no_ih(%rip),%r15
>          jz      x86_32_switch
>  
> +        push    %rax
> +        push    %rcx
> +        push    %rdx
> +        push    %rsi
> +        push    %rdi
> +
> +        /* Merge lower half of EFI pagtables with upper half of Xen 
> pagetables */
> +        mov     %cr3, %rsi
> +        lea     efi_l4_bootmap(%rip), %rdi
> +        mov     $L4_PAGETABLE_ENTRIES / 2, %ecx
> +        rep movsq
> +        lea     ((L4_PAGETABLE_ENTRIES / 2) * 8) + idle_pg_table(%rip), %rsi
> +        mov     $L4_PAGETABLE_ENTRIES / 2, %ecx
> +        rep movsq
> +
> +        /* Switch to merged pagetables */
> +        lea     efi_l4_bootmap(%rip), %rax
> +        mov     %rax, %cr3

While it may appear to work in some environments, I don't think we're allowed
to replace page tables prior to calling ExitBootServices(). IOW we may not
complain if somewhere this fails to work.

This also builds on the assumption that no new L4 entries would be made by
firmware, nor that it would put anything in the upper half of the L4 table.

> +        lea     __image_base__(%rip), %esi
> +
> +        /* Map Xen into the higher mappings using 2M superpages. */
> +        lea     _PAGE_PSE + PAGE_HYPERVISOR_RWX + sym_esi(_start), %eax

I'm surprised to see you put in place new RWX mappings, when the goal
supposedly is to get rid of any such.

> +        mov     $sym_offs(_start),   %ecx   /* %eax = PTE to write ^      */
> +        mov     $sym_offs(_end - 1), %edx
> +        shr     $L2_PAGETABLE_SHIFT, %ecx   /* %ecx = First slot to write */
> +        shr     $L2_PAGETABLE_SHIFT, %edx   /* %edx = Final slot to write */
> +
> +1:      mov     %eax, sym_offs(l2_xenmap)(%esi, %ecx, 8)
> +        add     $1, %ecx
> +        add     $1 << L2_PAGETABLE_SHIFT, %eax
> +
> +        cmp     %edx, %ecx
> +        jbe     1b
> +
> +        /*
> +         * Map Xen into the directmap (needed for early-boot pagetable
> +         * handling/walking), and identity map Xen into bootmap (needed for
> +         * the transition into long mode), using 2M superpages.
> +         */
> +        lea     sym_esi(_start), %ecx
> +        lea     -1 + sym_esi(_end), %edx
> +        lea     _PAGE_PSE + PAGE_HYPERVISOR_RWX(%ecx), %eax /* PTE to write. 
> */
> +        shr     $L2_PAGETABLE_SHIFT, %ecx                   /* First slot to 
> write. */
> +        shr     $L2_PAGETABLE_SHIFT, %edx                   /* Final slot to 
> write. */
> +
> +1:      mov     %eax, sym_offs(l2_bootmap)  (%esi, %ecx, 8)
> +        mov     %eax, sym_offs(l2_directmap)(%esi, %ecx, 8)
> +        add     $1, %ecx
> +        add     $1 << L2_PAGETABLE_SHIFT, %eax
> +
> +        cmp     %edx, %ecx
> +        jbe     1b
> +
> +        pop     %rdi
> +        pop     %rsi
> +        pop     %rdx
> +        pop     %rcx
> +        pop     %rax
> +
>          /* Save Multiboot2 magic on the stack. */
>          push    %rax
>  
> @@ -354,8 +414,15 @@ __efi64_mb2_start:
>           * efi_multiboot2() is called according to System V AMD64 ABI:
>           *   - IN:  %rdi - EFI ImageHandle, %rsi - EFI SystemTable,
>           *          %rdx - MB2 cmdline
> +         *
> +         * Call via the high mappings
>           */
> -        call    efi_multiboot2
> +        lea     __image_base__(%rip), %r10
> +        lea     efi_multiboot2(%rip), %rax
> +        sub     %r10, %rax
> +        mov     $__XEN_VIRT_START, %r10
> +        addq    %r10, %rax
> +        call    *%rax

While with this you arrange for all %rip-relative addressing to result in
using the linked-at positions, static pointers requiring base relocations
will still point at the loaded-at locations. Things would get particularly
interesting if the difference of two such (dissimilar) pointers would then
be calculated.

Jan



 


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