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Re: [Qemu-arm] [PATCH for-2.8] target-arm/translate-a64: fix gen_load_ex

From: Alex Bennée
Subject: Re: [Qemu-arm] [PATCH for-2.8] target-arm/translate-a64: fix gen_load_exclusive
Date: Mon, 05 Dec 2016 17:04:45 +0000
User-agent: mu4e 0.9.18; emacs

Richard Henderson <address@hidden> writes:

> On 12/05/2016 03:09 AM, Alex Bennée wrote:
>> Alex Bennée <address@hidden> writes:
>>> While testing rth's latest TCG patches with risu I found ldaxp was
>>> broken. Investigating further I found it was broken by 1dd089d0 when
>>> the cmpxchg atomic work was merged.
>> CC'ing Paolo/Richard
>> Do you guys have any final TCG fixes planned for 2.8 that can take this
>> fix for the ldaxp regression?
> I don't have any pending patchs for 2.8, no.
>>> As part of that change the code
>>> attempted to be clever by doing a single 64 bit load and then shuffle
>>> the data around to set the two 32 bit registers.
> It's not trying to be "clever", it's trying to be correct, giving an atomic
> 64-bit load.

Ahh right I see. What happens if the backend is 32bit, will it issue two
loads anyway?

> The fact that we didn't attempt an atomic 128-bit load here for the 64-bit 
> pair
> is a bug.  We have support for that in helper_atomic_ldo_le_mmu; I'm not sure
> why I didn't use that here.
>>> As I couldn't quite follow the endian magic I've simply partially
>>> reverted the change to the original code gen_load_exclusive code. This
>>> doesn't affect the cmpxchg functionality as that is all done on in
>>> gen_store_exclusive part which is untouched.
> We'll have to fix it properly eventually.  But perhaps 2.9 is soon enough,
> since that's when mttcg will go in.

I guess the easiest way would be to punt these paired loads to a helper?

However regardless of the atomicity the actual values loaded into
the registers are wrong so that behaviour is a regression vs 2.7.

I don't know how often these load-exclusive paired operations are used
in real code but I think we should at least fix it so the values are
loaded properly for 2.8 and do the proper atomic fix for 2.9.

> r~

Alex Bennée

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