Re: [Qemu-devel] Help on TLB Flush

[Peter Maydell]

On 13 February 2015 at 07:16, Mark Burton <address@hidden> wrote:
> If the kernel is doing this - then effectively - for X86, each CPU only
> flush’s it’s own TLB (from the perspective of Qemu) - correct?
> (in which case, for Qemu itself - for x86) - we dont need to implement
> a global flush, and hence we dont need to build the mechanism to sync ?

The semantics you need are "flush the QEMU TLB for CPU X" (where
X may not be the CPU you're running on). This is what tlb_flush()
does: it takes a CPU argument to act on. (Ditto tlb_flush_page, etc.)
We then use that to implement the target's required semantics
(eg in ARM the tlbiall_is_write() function is handled by iterating
through all CPUs and calling tlb_flush on them).

If you don't want the pain of checking the semantics of every
backend and figuring out a new set of primitives to implement,
then what you need to do is continue to provide the guarantees
the current tlb_flush function does: when it returns then the
CPU it's supposed to have acted on has definitely done so.

You can try and be cleverer if you want to, but personally
I would recommend keeping the scope of your work simple
where you can.

-- PMM