Re: [PATCH v7 2/5] xen/arm64: Rework the memory layout

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Julien,

> On 16 Apr 2023, at 16:32, Julien Grall <julien@xxxxxxx> wrote:
> 
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> Xen is currently not fully compliant with the Arm Arm because it will
> switch the TTBR with the MMU on.
> 
> In order to be compliant, we need to disable the MMU before
> switching the TTBR. The implication is the page-tables should
> contain an identity mapping of the code switching the TTBR.
> 
> In most of the case we expect Xen to be loaded in low memory. I am aware
> of one platform (i.e AMD Seattle) where the memory start above 512GB.
> To give us some slack, consider that Xen may be loaded in the first 2TB
> of the physical address space.
> 
> The memory layout is reshuffled to keep the first four slots of the zeroeth
> level free. All the regions currently in L0 slot 0 will not be part of
> slot 4 (2TB). This requires a slight tweak of the boot code because
> XEN_VIRT_START (2TB + 2MB) cannot be used as an immediate.
> 
> This reshuffle will make trivial to create a 1:1 mapping when Xen is
> loaded below 2TB.
> 
> Lastly, take the opportunity to check a compile time if any of the
> regions may overlap with the reserved area for identity mapping.
> 
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

With the 2 typos found by Michal fixed:
Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>

Cheers
Bertrand

> 
> ----
>    Changes in v7:
>        - Remove all tags
>        - Add BUILD_BUG_ON()s
>        - Don't forget to update FRAMETABLE_VIRT_START and
>          VMAP_VIRT_START
> 
>    Changes in v6:
>        - Correct the BUILD_BUG_ON(), Xen virtual address should be
>          above 2TB (i.e. slot0 > 4).
>        - Add Bertrand's reviewed-by
> 
>    Changes in v5:
>        - We are reserving 4 slots rather than 2.
>        - Fix the addresses in the layout comment.
>        - Fix the size of the region in the layout comment
>        - Add Luca's tested-by (the reviewed-by was not added
>          because of the changes requested by Michal)
>        - Add Michal's reviewed-by
> 
>    Changes in v4:
>        - Correct the documentation
>        - The start address is 2TB, so slot0 is 4 not 2.
> 
>    Changes in v2:
>        - Reword the commit message
>        - Load Xen at 2TB + 2MB
>        - Update the documentation to reflect the new layout
> ---
> xen/arch/arm/arm64/head.S         |  3 ++-
> xen/arch/arm/include/asm/config.h | 38 +++++++++++++++++++++----------
> xen/arch/arm/mm.c                 | 23 +++++++++++++++----
> 3 files changed, 46 insertions(+), 18 deletions(-)
> 
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index 4a3f87117c83..663f5813b12e 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -607,7 +607,8 @@ create_page_tables:
>          * need an additional 1:1 mapping, the virtual mapping will
>          * suffice.
>          */
> -        cmp   x19, #XEN_VIRT_START
> +        ldr   x0, =XEN_VIRT_START
> +        cmp   x19, x0
>         bne   1f
>         ret
> 1:
> diff --git a/xen/arch/arm/include/asm/config.h 
> b/xen/arch/arm/include/asm/config.h
> index 5df0e4c4959b..2cfe5e480256 100644
> --- a/xen/arch/arm/include/asm/config.h
> +++ b/xen/arch/arm/include/asm/config.h
> @@ -72,16 +72,13 @@
> #include <xen/page-size.h>
> 
> /*
> - * Common ARM32 and ARM64 layout:
> + * ARM32 layout:
>  *   0  -   2M   Unmapped
>  *   2M -   4M   Xen text, data, bss
>  *   4M -   6M   Fixmap: special-purpose 4K mapping slots
>  *   6M -  10M   Early boot mapping of FDT
>  *   10M - 12M   Livepatch vmap (if compiled in)
>  *
> - * ARM32 layout:
> - *   0  -  12M   <COMMON>
> - *
>  *  32M - 128M   Frametable: 32 bytes per page for 12GB of RAM
>  * 256M -   1G   VMAP: ioremap and early_ioremap use this virtual address
>  *                    space
> @@ -90,14 +87,23 @@
>  *   2G -   4G   Domheap: on-demand-mapped
>  *
>  * ARM64 layout:
> - * 0x0000000000000000 - 0x0000007fffffffff (512GB, L0 slot [0])
> - *   0  -  12M   <COMMON>
> + * 0x0000000000000000 - 0x000001ffffffffff (2TB, L0 slots [0..3])
> + *
> + *  Reserved to identity map Xen
> + *
> + * 0x0000020000000000 - 0x0000027fffffffff (512GB, L0 slot [4]
> + *  (Relative offsets)
> + *   0  -   2M   Unmapped
> + *   2M -   4M   Xen text, data, bss
> + *   4M -   6M   Fixmap: special-purpose 4K mapping slots
> + *   6M -  10M   Early boot mapping of FDT
> + *  10M -  12M   Livepatch vmap (if compiled in)
>  *
>  *   1G -   2G   VMAP: ioremap and early_ioremap
>  *
>  *  32G -  64G   Frametable: 56 bytes per page for 2TB of RAM
>  *
> - * 0x0000008000000000 - 0x00007fffffffffff (127.5TB, L0 slots [1..255])
> + * 0x0000028000000000 - 0x00007fffffffffff (125TB, L0 slots [5..255])
>  *  Unused
>  *
>  * 0x0000800000000000 - 0x000084ffffffffff (5TB, L0 slots [256..265])
> @@ -107,7 +113,17 @@
>  *  Unused
>  */
> 
> +#ifdef CONFIG_ARM_32
> #define XEN_VIRT_START          _AT(vaddr_t, MB(2))
> +#else
> +
> +#define SLOT0_ENTRY_BITS  39
> +#define SLOT0(slot) (_AT(vaddr_t,slot) << SLOT0_ENTRY_BITS)
> +#define SLOT0_ENTRY_SIZE  SLOT0(1)
> +
> +#define XEN_VIRT_START          (SLOT0(4) + _AT(vaddr_t, MB(2)))
> +#endif
> +
> #define XEN_VIRT_SIZE           _AT(vaddr_t, MB(2))
> 
> #define FIXMAP_VIRT_START       (XEN_VIRT_START + XEN_VIRT_SIZE)
> @@ -163,14 +179,12 @@
> 
> #else /* ARM_64 */
> 
> -#define SLOT0_ENTRY_BITS  39
> -#define SLOT0(slot) (_AT(vaddr_t,slot) << SLOT0_ENTRY_BITS)
> -#define SLOT0_ENTRY_SIZE  SLOT0(1)
> +#define IDENTITY_MAPPING_AREA_NR_L0  4
> 
> -#define VMAP_VIRT_START  GB(1)
> +#define VMAP_VIRT_START  (SLOT0(4) + GB(1))
> #define VMAP_VIRT_SIZE   GB(1)
> 
> -#define FRAMETABLE_VIRT_START  GB(32)
> +#define FRAMETABLE_VIRT_START  (SLOT0(4) + GB(32))
> #define FRAMETABLE_SIZE        GB(32)
> #define FRAMETABLE_NR          (FRAMETABLE_SIZE / sizeof(*frame_table))
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index b99806af996c..1d09d61dd922 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -153,7 +153,19 @@ static void __init __maybe_unused build_assertions(void)
> #endif
>     /* Page table structure constraints */
> #ifdef CONFIG_ARM_64
> -    BUILD_BUG_ON(zeroeth_table_offset(XEN_VIRT_START));
> +    /*
> +     * The first few slots of the L0 table is reserved for the identity
> +     * mapping. Check that none of the other regions are overlapping
> +     * with it.
> +     */
> +#define CHECK_OVERLAP_WITH_IDMAP(virt) \
> +    BUILD_BUG_ON(zeroeth_table_offset(virt) < IDENTITY_MAPPING_AREA_NR_L0)
> +
> +    CHECK_OVERLAP_WITH_IDMAP(XEN_VIRT_START);
> +    CHECK_OVERLAP_WITH_IDMAP(VMAP_VIRT_START);
> +    CHECK_OVERLAP_WITH_IDMAP(FRAMETABLE_VIRT_START);
> +    CHECK_OVERLAP_WITH_IDMAP(DIRECTMAP_VIRT_START);
> +#undef CHECK_OVERLAP_WITH_IDMAP
> #endif
>     BUILD_BUG_ON(first_table_offset(XEN_VIRT_START));
> #ifdef CONFIG_ARCH_MAP_DOMAIN_PAGE
> @@ -496,10 +508,11 @@ void __init setup_pagetables(unsigned long 
> boot_phys_offset)
>     phys_offset = boot_phys_offset;
> 
> #ifdef CONFIG_ARM_64
> -    p = (void *) xen_pgtable;
> -    p[0] = pte_of_xenaddr((uintptr_t)xen_first);
> -    p[0].pt.table = 1;
> -    p[0].pt.xn = 0;
> +    pte = pte_of_xenaddr((uintptr_t)xen_first);
> +    pte.pt.table = 1;
> +    pte.pt.xn = 0;
> +    xen_pgtable[zeroeth_table_offset(XEN_VIRT_START)] = pte;
> +
>     p = (void *) xen_first;
> #else
>     p = (void *) cpu0_pgtable;
> -- 
> 2.39.2
>