On Fri, Jul 08, 2022 at 06:35:48PM +0000, Nadav Amit wrote: > 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”. Stupid example that doesn't help. > > 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. Well now, that's some bullshit. I did my fucking research. I went back 14+ years in history to figure out what was going on back then. I cited commit IDs. You're just tossing off some opinions. I have no misconceptions about whatever you want to call the mechanism for communicating with the hypervisor at a higher level than "prod this byte". For example, one of the more intensive things we use config space for is sizing BARs. If we had a hypercall to siz a BAR, that would eliminate: - Read current value from BAR - Write all-ones to BAR - Read new value from BAR - Write original value back to BAR Bingo, one hypercall instead of 4 MMIO or 8 PIO accesses. Just because I don't use your terminology, you think I have "misconceptions"? Fuck you, you condescending piece of shit. > 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. And yet, no numbers. Yes, microbenchmark numbers that provde nothing, but no numbers about how much it improves boot time. > 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. >
