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

[Aurelien Jarno]

On 2015-07-21 14:04, Alexander Graf wrote:
> 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 <address@hidden>
> >Thanks.
> 
> So who can pick those up? PPC TCG is pretty much unmaintained since Malc
> disappeared, no?
> 

Richard has reviewed this patch, and there are already generic TCG patches
for 2.5 on the list. I guess he can squeeze a few PPC specific commits
like this one in the pull request.

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
address@hidden                 http://www.aurel32.net