Re: [Qemu-devel] [PATCH v3] tcg/ppc: Improve unaligned load/store handling on 64-bit backend

[Alexander Graf <agraf@suse.de>]

On 07/21/15 11:46, Benjamin Herrenschmidt wrote:
> On Tue, 2015-07-21 at 07:39 +0100, Richard Henderson wrote:
>> On 07/21/2015 07:33 AM, Benjamin Herrenschmidt wrote:
>>> On Tue, 2015-07-21 at 07:27 +0100, Richard Henderson wrote:
>>>> On 07/21/2015 06:19 AM, Benjamin Herrenschmidt wrote:
>>>>> +    /* Clear the non-page, non-alignment bits from the address */
>>>>>         if (TCG_TARGET_REG_BITS == 32 || TARGET_LONG_BITS == 32) {
>>>>> +        /* We don't support unaligned accesses on 32-bits, preserve
>>>>> +         * the bottom bits and thus trigger a comparison failure on
>>>>> +         * unaligned accesses
>>>>> +         */
>>>>>             tcg_out_rlw(s, RLWINM, TCG_REG_R0, addrlo, 0,
>>>>>                         (32 - s_bits) & 31, 31 - TARGET_PAGE_BITS);
>>>> Why don't you support this unaligned acess with 32-bit guests?
>>> No reason, I just didn't get to do it yet. It's possible, I was just
>>> lazy :-) It also adds one instruction. On 64-bit we always have 2
>>> instructions anyway so it wasn't adding any overhead really, on 32-bit
>>> we get away with a single rlwinm, while adding the unaligned support
>>> would make it an addi + rlwinm.
>> Ah, ok.  I wondered if some older 32-bit host ppc didn't allow it, and the
>> 32-bit guest paid the price.  Anyway,
> No, most implementations support unaligned accesses in common cases. I
> think the worst embedded variants might trap when crossing page
> boundaries but that's about it. I don't think we bother emulating
> this though.
>
>> Reviewed-by: Richard Henderson <rth@twiddle.net>
> Thanks.

So who can pick those up? PPC TCG is pretty much unmaintained since Malc 
disappeared, no?


Alex