RE: [PATCH] xen/arm: Remove most of the *_VIRT_END defines

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Julien,

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
> Julien Grall
> Sent: 2022年5月24日 3:50
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: julien@xxxxxxx; Julien Grall <jgrall@xxxxxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Bertrand Marquis <Bertrand.Marquis@xxxxxxx>;
> Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; Konrad Rzeszutek Wilk
> <konrad.wilk@xxxxxxxxxx>; Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> Subject: [PATCH] xen/arm: Remove most of the *_VIRT_END defines
> 
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> At the moment, *_VIRT_END may either point to the address after the end
> or the last address of the region.
> 
> The lack of consistency make quite difficult to reason with them.
> 
> Furthermore, there is a risk of overflow in the case where the address
> points past to the end. I am not aware of any cases, so this is only a
> latent bug.
> 
> Start to solve the problem by removing all the *_VIRT_END exclusively used
> by the Arm code and add *_VIRT_SIZE when it is not present.
> 
> Take the opportunity to rename BOOT_FDT_SLOT_SIZE to BOOT_FDT_VIRT_SIZE
> for better consistency and use _AT(vaddr_t, ).
> 
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> 
> ----
> 
> I noticed that a few functions in Xen expect [start, end[. This is risky
> as we may end up with end < start if the region is defined right at the
> top of the address space.
> 
> I haven't yet tackle this issue. But I would at least like to get rid
> of *_VIRT_END.
> ---
>  xen/arch/arm/include/asm/config.h | 18 ++++++++----------
>  xen/arch/arm/livepatch.c          |  2 +-
>  xen/arch/arm/mm.c                 | 13 ++++++++-----
>  3 files changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/xen/arch/arm/include/asm/config.h
> b/xen/arch/arm/include/asm/config.h
> index 3e2a55a91058..66db618b34e7 100644
> --- a/xen/arch/arm/include/asm/config.h
> +++ b/xen/arch/arm/include/asm/config.h
> @@ -111,12 +111,11 @@
>  #define FIXMAP_ADDR(n)        (_AT(vaddr_t,0x00400000) + (n) * PAGE_SIZE)
> 
>  #define BOOT_FDT_VIRT_START    _AT(vaddr_t,0x00600000)
> -#define BOOT_FDT_SLOT_SIZE     MB(4)
> -#define BOOT_FDT_VIRT_END      (BOOT_FDT_VIRT_START + BOOT_FDT_SLOT_SIZE)
> +#define BOOT_FDT_VIRT_SIZE     _AT(vaddr_t, MB(4))
> 
>  #ifdef CONFIG_LIVEPATCH
>  #define LIVEPATCH_VMAP_START   _AT(vaddr_t,0x00a00000)
> -#define LIVEPATCH_VMAP_END     (LIVEPATCH_VMAP_START + MB(2))
> +#define LIVEPATCH_VMAP_SIZE    _AT(vaddr_t, MB(2))
>  #endif
> 
>  #define HYPERVISOR_VIRT_START  XEN_VIRT_START
> @@ -132,18 +131,18 @@
>  #define FRAMETABLE_VIRT_END    (FRAMETABLE_VIRT_START + FRAMETABLE_SIZE -
> 1)
> 
>  #define VMAP_VIRT_START        _AT(vaddr_t,0x10000000)
> +#define VMAP_VIRT_SIZE         _AT(vaddr_t, GB(1) - MB(256))
> 
>  #define XENHEAP_VIRT_START     _AT(vaddr_t,0x40000000)
> -#define XENHEAP_VIRT_END       _AT(vaddr_t,0x7fffffff)
> -#define DOMHEAP_VIRT_START     _AT(vaddr_t,0x80000000)
> -#define DOMHEAP_VIRT_END       _AT(vaddr_t,0xffffffff)
> +#define XENHEAP_VIRT_SIZE      _AT(vaddr_t, GB(1))
> 
> -#define VMAP_VIRT_END    XENHEAP_VIRT_START
> +#define DOMHEAP_VIRT_START     _AT(vaddr_t,0x80000000)
> +#define DOMHEAP_VIRT_SIZE      _AT(vaddr_t, GB(2))
> 
>  #define DOMHEAP_ENTRIES        1024  /* 1024 2MB mapping slots */
> 
>  /* Number of domheap pagetable pages required at the second level (2MB
> mappings) */
> -#define DOMHEAP_SECOND_PAGES ((DOMHEAP_VIRT_END - DOMHEAP_VIRT_START +
> 1) >> FIRST_SHIFT)
> +#define DOMHEAP_SECOND_PAGES (DOMHEAP_VIRT_SIZE >> FIRST_SHIFT)
> 
>  #else /* ARM_64 */
> 
> @@ -152,12 +151,11 @@
>  #define SLOT0_ENTRY_SIZE  SLOT0(1)
> 
>  #define VMAP_VIRT_START  GB(1)
> -#define VMAP_VIRT_END    (VMAP_VIRT_START + GB(1))
> +#define VMAP_VIRT_SIZE   GB(1)
> 
>  #define FRAMETABLE_VIRT_START  GB(32)
>  #define FRAMETABLE_SIZE        GB(32)
>  #define FRAMETABLE_NR          (FRAMETABLE_SIZE / sizeof(*frame_table))
> -#define FRAMETABLE_VIRT_END    (FRAMETABLE_VIRT_START + FRAMETABLE_SIZE -
> 1)
> 
>  #define DIRECTMAP_VIRT_START   SLOT0(256)
>  #define DIRECTMAP_SIZE         (SLOT0_ENTRY_SIZE * (265-256))
> diff --git a/xen/arch/arm/livepatch.c b/xen/arch/arm/livepatch.c
> index 75e8adcfd6a1..57abc746e60b 100644
> --- a/xen/arch/arm/livepatch.c
> +++ b/xen/arch/arm/livepatch.c
> @@ -175,7 +175,7 @@ void __init arch_livepatch_init(void)
>      void *start, *end;
> 
>      start = (void *)LIVEPATCH_VMAP_START;
> -    end = (void *)LIVEPATCH_VMAP_END;
> +    end = start + LIVEPATCH_VMAP_SIZE;
> 
>      vm_init_type(VMAP_XEN, start, end);
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index be37176a4725..0607c65f95cd 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -128,9 +128,11 @@ static DEFINE_PAGE_TABLE(xen_first);
>  /* xen_pgtable == root of the trie (zeroeth level on 64-bit, first on 32-
> bit) */
>  static DEFINE_PER_CPU(lpae_t *, xen_pgtable);
>  #define THIS_CPU_PGTABLE this_cpu(xen_pgtable)
> -/* xen_dommap == pages used by map_domain_page, these pages contain
> +/*
> + * xen_dommap == pages used by map_domain_page, these pages contain
>   * the second level pagetables which map the domheap region
> - * DOMHEAP_VIRT_START...DOMHEAP_VIRT_END in 2MB chunks. */
> + * starting at DOMHEAP_VIRT_START in 2MB chunks.
> + */
>  static DEFINE_PER_CPU(lpae_t *, xen_dommap);
>  /* Root of the trie for cpu0, other CPU's PTs are dynamically allocated
> */
>  static DEFINE_PAGE_TABLE(cpu0_pgtable);
> @@ -476,7 +478,7 @@ mfn_t domain_page_map_to_mfn(const void *ptr)
>      int slot = (va - DOMHEAP_VIRT_START) >> SECOND_SHIFT;
>      unsigned long offset = (va>>THIRD_SHIFT) & XEN_PT_LPAE_ENTRY_MASK;
> 
> -    if ( va >= VMAP_VIRT_START && va < VMAP_VIRT_END )
> +    if ( (va >= VMAP_VIRT_START) && ((VMAP_VIRT_START - va) <
> VMAP_VIRT_SIZE) )
>          return virt_to_mfn(va);
> 
>      ASSERT(slot >= 0 && slot < DOMHEAP_ENTRIES);
> @@ -570,7 +572,8 @@ void __init remove_early_mappings(void)
>      int rc;
> 
>      /* destroy the _PAGE_BLOCK mapping */
> -    rc = modify_xen_mappings(BOOT_FDT_VIRT_START, BOOT_FDT_VIRT_END,
> +    rc = modify_xen_mappings(BOOT_FDT_VIRT_START,
> +                             BOOT_FDT_VIRT_START + BOOT_FDT_VIRT_SIZE,
>                               _PAGE_BLOCK);
>      BUG_ON(rc);
>  }
> @@ -850,7 +853,7 @@ void __init setup_frametable_mappings(paddr_t ps,
> paddr_t pe)
> 
>  void *__init arch_vmap_virt_end(void)
>  {
> -    return (void *)VMAP_VIRT_END;
> +    return (void *)(VMAP_VIRT_START + VMAP_VIRT_SIZE);

It seems you prefer to point _end to the address after the end. Even
though we got rid of the macro definition of _END. But we didn't agree
on how to use it. For me, when I first saw
"end = start + LIVEPATCH_VMAP_SIZE" I subconsciously think the -1 is
missing here. I even added a comment, but removed it when I reached
to this line : )
May be it's better to place some code guide for END in code comment
in the SIZE definition, otherwise, we may have different point addresses
of _end functions.

Cheers,
Wei Chen

>  }
> 
>  /*
> --
> 2.32.0
>