Re: [Qemu-devel] How expensive is an extra QEMU TLB MMU index ?

[Richard Henderson]

On 04/21/2017 10:23 AM, Peter Maydell wrote:
> Hi; I have a feature for the ARM M profile emulation which I think
> needs to be implemented by giving it a separate mmu index value.
> [The HFNMIENA control bit allows "privileged and execution priority
> is higher than 0" to run with the MPU effectively off even if normal
> privileged execution has enabled the MPU.]
>
> I could implement this by just bumping ARM's NB_MMU_MODES to 8
> and using the extra value for this. On the other hand, several
> of the current 7 index values are not needed for M profile CPUs,
> so in theory I could reuse one of those (so it would have different
> meanings if the CPU was M profile or not). But that feels like it would
> be pretty accident-prone, since so much of the code is shared.
>
> So, just how expensive is having an extra MMU mode which isn't
> actually used? Is it bad enough to be worth the reduced
> maintainability?

Once you get above 7 indicies, we will start to reduce the size of the TLB for 
various hosts.  See CPU_TLB_BITS in include/exec/cpu-defs.h.

That said, the size reduction doesn't happen for x86_64 host, and ought not 
actively fail for any host, so maybe you don't care.


r~