Re: [Qemu-devel] [PATCH 2/2] exec: make address spaces 64-bit wide

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 07, 2013 at 05:14:37PM +0100, Paolo Bonzini wrote:
> As an alternative to commit 818f86b (exec: limit system memory
> size, 2013-11-04) let's just make all address spaces 64-bit wide.
> This eliminates problems with phys_page_find ignoring bits above
> TARGET_PHYS_ADDR_SPACE_BITS and address_space_translate_internal
> consequently messing up the computations.
> 
> In Luiz's reported crash, at startup gdb attempts to read from address
> 0xffffffffffffffe6 to 0xffffffffffffffff inclusive.  The region it gets
> is the newly introduced master abort region, which is as big as the PCI
> address space (see pci_bus_init).  Due to a typo that's only 2^63-1,
> not 2^64.  But we get it anyway because phys_page_find ignores the upper
> bits of the physical address.  In address_space_translate_internal then
> 
>     diff = int128_sub(section->mr->size, int128_make64(addr));
>     *plen = int128_get64(int128_min(diff, int128_make64(*plen)));
> 
> diff becomes negative, and int128_get64 booms.
> 
> The size of the PCI address space region should be fixed anyway.
> 
> Reported-by: Luiz Capitulino <lcapitulino@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

So this causes a 12% performance regression on some TCG
tests, I think we should look into a smarter
datastructure to solve the issues.

> ---
>  exec.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/exec.c b/exec.c
> index 9e2fc4b..d5ce3da 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -89,7 +89,7 @@ struct PhysPageEntry {
>  };
>  
>  /* Size of the L2 (and L3, etc) page tables.  */
> -#define ADDR_SPACE_BITS TARGET_PHYS_ADDR_SPACE_BITS
> +#define ADDR_SPACE_BITS 64
>  
>  #define P_L2_BITS 10
>  #define P_L2_SIZE (1 << P_L2_BITS)
> @@ -1750,11 +1750,7 @@ static void memory_map_init(void)
>  {
>      system_memory = g_malloc(sizeof(*system_memory));
>  
> -    assert(ADDR_SPACE_BITS <= 64);
> -
> -    memory_region_init(system_memory, NULL, "system",
> -                       ADDR_SPACE_BITS == 64 ?
> -                       UINT64_MAX : (0x1ULL << ADDR_SPACE_BITS));
> +    memory_region_init(system_memory, NULL, "system", UINT64_MAX);
>      address_space_init(&address_space_memory, system_memory, "memory");
>  
>      system_io = g_malloc(sizeof(*system_io));
> -- 
> 1.8.4.2
>