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Re: [Qemu-devel] [PATCH] cpu_defs: Simplify CPUTLB padding logic
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] cpu_defs: Simplify CPUTLB padding logic |
Date: |
Mon, 6 Jul 2015 10:43:18 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.0.1 |
On 05/07/2015 23:08, Peter Crosthwaite wrote:
> There was a complicated subtractive arithmetic for determining the
> padding on the CPUTLBEntry structure. Simplify this with a union.
>
> Signed-off-by: Peter Crosthwaite <address@hidden>
> ---
> include/exec/cpu-defs.h | 23 ++++++++++++-----------
> 1 file changed, 12 insertions(+), 11 deletions(-)
>
> diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h
> index 98b9cff..5093be2 100644
> --- a/include/exec/cpu-defs.h
> +++ b/include/exec/cpu-defs.h
> @@ -105,17 +105,18 @@ typedef struct CPUTLBEntry {
> bit 3 : indicates that the entry is invalid
> bit 2..0 : zero
> */
> - target_ulong addr_read;
> - target_ulong addr_write;
> - target_ulong addr_code;
> - /* Addend to virtual address to get host address. IO accesses
> - use the corresponding iotlb value. */
> - uintptr_t addend;
> - /* padding to get a power of two size */
> - uint8_t dummy[(1 << CPU_TLB_ENTRY_BITS) -
> - (sizeof(target_ulong) * 3 +
> - ((-sizeof(target_ulong) * 3) & (sizeof(uintptr_t) - 1)) +
> - sizeof(uintptr_t))];
> + union {
The struct CPUTLBEntry can be changed to union CPUTLBEntry directly,
with no need for the anonymous struct.
> + struct {
> + target_ulong addr_read;
> + target_ulong addr_write;
> + target_ulong addr_code;
> + /* Addend to virtual address to get host address. IO accesses
> + use the corresponding iotlb value. */
> + uintptr_t addend;
> + };
Which compiler version started implementing anonymous structs?
Or can we just add
__attribute__((__aligned__(1 << CPU_TLB_ENTRY_BITS)))
to the struct? I'm not sure if it affects the sizeof too, so that
requires some care. Alternatively, an
uint8_t padding[0]
__attribute__((__aligned__(1 << CPU_TLB_ENTRY_BITS)));
could maybe work? Neither is exactly the same, as they also bump the
alignment of the overall struct, but they do not require anonymous structs.
Paolo
> + /* padding to get a power of two size */
> + uint8_t dummy[1 << CPU_TLB_ENTRY_BITS];
> + };
> } CPUTLBEntry;
>
> QEMU_BUILD_BUG_ON(sizeof(CPUTLBEntry) != (1 << CPU_TLB_ENTRY_BITS));
>