Re: [Qemu-devel] [RFC v5 1/6] exec.c: Add new exclusive bitmap to ram_list

[Richard Henderson]

On 09/24/2015 01:32 AM, Alvise Rigo wrote:
> +    if (cpu == smp_cpus) {
> +        if (smp_cpus >= EXCL_BITMAP_CELL_SZ) {
> +            return bitmap[EXCL_BITMAP_GET_BYTE_OFFSET(addr)];
> +        } else {
> +            return bitmap[EXCL_BITMAP_GET_BYTE_OFFSET(addr)] &
> +                                            ((1 << smp_cpus) - 1);
> +        }
> +    } else {
> +        return bitmap[EXCL_BITMAP_GET_BYTE_OFFSET(addr)] & (1 << 
> EXCL_IDX(cpu));
> +    }

How can more than one cpu have the same address exclusively?

Isn't this scheme giving a whole page to a cpu, not a cacheline?
That's going to cause ll/sc conflicts where real hardware wouldn't.



r~