On Jul 8, 2022, at 10:55 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > ⚠ External Email > > 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. Wild example, hence the “for instance”. > > 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? Well, you used a lot of words to say “it causes problems” without saying which. It appeared you have misconceptions about paravirtualization that I wanted to correct. As I said before, I am not familiar with the exact motivation for this patch. I now understood from Ajay that it shortens VM boot time considerably. I was talking to Ajay to see if there is a possibility of a VMware specific solution. I am afraid that init_hypervisor_platform() might take place too late.
