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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
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Date: Mon, 21 Sep 2015 21:59:03 +0100
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Subject: Re: [Qemu-devel] [RFC 17/38] aie: add target helpers
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: mttcg@listserver.greensocs.com, mark.burton@greensocs.com, qemu-devel@nongnu.org, a.rigo@virtualopensystems.com, "Emilio G. Cota" <cota@braap.org>, guillaume.delbergue@greensocs.com, Frederic Konrad <fred.konrad@greensocs.com>


Paolo Bonzini <pbonzini@redhat.com> writes:

> On 24/08/2015 02:23, Emilio G. Cota wrote:
>> +void HELPER(aie_st_pre)(CPUArchState *env, target_ulong vaddr)
>> +{
>> +    if (unlikely(env->aie_lock_enabled)) {
>> +        if (env->aie_locked) {
>> +            return;
>> +        }
>
> Now that I've reviewed your code more carefully, the approach you're
> using looks more promising than I thought.  There are advantages over
> Alvise's code, namely:
>
> - cache-line vs. page granularity

There is nothing that stops Alvise's slow-path helpers having a better
granularity when deciding if the exclusive flag needs tripping. However
I like the lookup approach in these patches.

> - avoiding the global TLB flush (due to the virtually indexed TLBs)
>
> - easy support for user-mode emulation
>
> and some of the disadvantages look more easily fixable than I thought,
> too (e.g. TLB entry bloat).
>
> The main advantage of Alvise's code, on the other hand, is the minimal
> overhead when there are no active LL/SC combinations and the better
> integration with TCG.

I too prefer the expression of LL/SC semantics via TCG ops. 

> A random idea: would it be possible to move some of the helper code to
> generated TCG code?  For example, maintaining a count of outstanding
> load-locked operations and forcing the slow path for stores if it is
> non-zero?
>
> Paolo
>
>> +        aie_st_lock_ret(env, vaddr, GETRA());
>> +    } else {
>> +        hwaddr paddr = h_get_st_phys(env, vaddr, GETRA());
>> +
>> +        if (unlikely(aie_entry_exists(paddr))) {
>> +            h_aie_lock(env, paddr);
>> +        }
>> +    }
>> +}

-- 
Alex Bennée