Re: [Qemu-devel] [PATCH v4 25/26] tcg: Check for overflow via highwater mark

[Peter Maydell]

On 30 September 2015 at 17:50, Aurelien Jarno <address@hidden> wrote:
> On 2015-09-30 15:09, Richard Henderson wrote:
>> We currently pre-compute an worst case code size for any TB, which
>> works out to be 122kB.  Since the average TB size is near 1kB, this
>> wastes quite a lot of storage.
>
> The code generation buffer is currently computed as 1/4 of the guest
> RAM in softmmu mode (so 32MB for the default 128MB of RAM) or 32MB in
> user mode. 122kB is therefore less than 0.4% of waster memory, I am not
> therefore sure we need to add so much code just for that.

I think the best argument for this patch is that it gets rid
of the TCG_MAX_OP_SIZE guesswork about TCG op worst-cases.
That sort of #define is generally arbitrary and liable to not
get updated when new targets or backends invalidate previous
assumptions.

(Can we do anything to get rid of MAX_OP_PER_INSTR too?)

thanks
-- PMM