Re: [Qemu-devel] [RFC 14/38] softmmu: add helpers to get ld/st physical addresses

[Emilio G. Cota]

On Sun, Aug 23, 2015 at 19:02:30 -0700, Paolo Bonzini wrote:
> On 23/08/2015 17:23, Emilio G. Cota wrote:
> > This will be used by the atomic instruction emulation code.
> 
> Is this a fast path?  If not, we can use the existing addend field and
> convert the host address to a ram_addr_t easily.

On x86 this is a fast path because the helper that goes before every
store (aie_st_pre, p.17/38) checks whether any atomic operations on
the store's cache line have been done before. The check is done using
a bitmap (p.16/38); the input to the bitmap is the physical address with
the last 6 bits shifted out (I'm assuming 2**6=64-byte cache lines).

That said: How would the conversion via the addend field look like?
If it's just an addition it might be cheap enough--and we wouldn't
bloat CPUTLBEntry.

Thanks,

                Emilio