Re: [Qemu-devel] [PATCH] cpu_defs: Simplify CPUTLB padding logic

[Richard Henderson]

On 07/06/2015 09:43 AM, Paolo Bonzini wrote:
> 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.

Um, no it can't.  That would put all of the members at the same address.

> Which compiler version started implementing anonymous structs?

A long long time ago -- gcc 2 era.

> Or can we just add
>
>          __attribute__((__aligned__(1 << CPU_TLB_ENTRY_BITS)))

The structure isn't currently aligned, and it needn't be.  We only need the 
size to be a power of two for the addressing.



r~