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From: Richard Henderson <rth@twiddle.net>
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Date: Mon, 17 Jul 2017 19:25:14 -1000
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Subject: Re: [Qemu-devel] [PATCH v2 45/45] tcg: enable multiple TCG contexts
 in softmmu
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To: "Emilio G. Cota" <cota@braap.org>, qemu-devel@nongnu.org

On 07/16/2017 10:04 AM, Emilio G. Cota wrote:
> +
> +    /* claim the first free pointer in tcg_ctxs and increment n_tcg_ctxs */
> +    for (i = 0; i < smp_cpus; i++) {
> +        if (atomic_cmpxchg(&tcg_ctxs[i], NULL, s) == NULL) {
> +            unsigned int n;
> +
> +            n = atomic_fetch_inc(&n_tcg_ctxs);

Surely this is too much effort.  The increment on n_tcg_ctxs is sufficient to 
produce an index for assignment.  We never free the contexts...

Which also suggests that it might be better to avoid an indirection in tcg_ctxs 
and allocate all of the structures in one big block?  I.e.

TCGContext *tcg_ctxs;

// At the end of tcg_context_init.
#ifdef CONFIG_USER_ONLY
     tcg_ctxs = s;
#else
     // No need to zero; we'll completely overwrite each structure
     // during tcg_register_thread.
     tcg_ctxs = g_new(TCGContext, smp_cpus);
#endif


r~