On Fri, Jul 08, 2022 at 04:45:00PM +0000, Nadav Amit wrote: > On Jul 8, 2022, at 5:56 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > And looking at the results above, it's not so much the PIO vs MMIO > > that makes a difference, it's the virtualisation. A mmio access goes > > from 269ns to 85us. Rather than messing around with preferring MMIO > > over PIO for config space, having an "enlightenment" to do config > > space accesses would be a more profitable path. > > I am unfamiliar with the motivation for this patch, but I just wanted to > briefly regard the advice about enlightments. > > “enlightenment”, AFAIK, is Microsoft’s term for "para-virtualization", so > let’s regard the generic term. I think that you consider the bare-metal > results as the possible results from a paravirtual machine, which is mostly > wrong. Para-virtualization usually still requires a VM-exit and for the most > part the hypervisor/host runs similar code for MMIO/hypercall (conceptually; > the code of paravirtual and fully-virtual devices is often different, but > IIUC, this is not what Ajay measured). > > Para-virtualization could have *perhaps* helped to reduce the number of > PIO/MMIO and improve performance this way. If, for instance, all the > PIO/MMIO are done during initialization, a paravirtual interface can be use > to batch them together, and that would help. But it is more complicated to > get a performance benefit from paravirtualization if the PIO/MMIO accesses > are “spread”, for instance, done after each interrupt. What kind of lousy programming interface requires you to do a config space access after every interrupt? This is looney-tunes. You've used a lot of words to not answer the question that was so important that I asked it twice. What's the use case, what's the workload that would benefit from this patch? > Para-virtauilzation and full-virtualization both have pros and cons. > Para-virtualization is many times more efficient, but requires the VM to > have dedicated device drivers for the matter. Try to run a less-common OS > than Linux and it would not work since the OS would not have drivers for the > paras-virtual devices. And even if you add support today for a para-virtual > devices, there are many deployed OSes that do not have such support, and you > would not be able to run them in a VM. > > Regardless to virtualization, Ajay’s results show PIO is slower on > bare-metal, and according to his numbers by 165ns, which is significant. > Emulating PIO in hypervisors on x86 is inherently more complex than MMIO, so > the results he got would most likely happen on all hypervisors. > > tl;dr: Let’s keep this discussion focused and put paravirtualization aside. > It is not a solution for all the problems in the world.
