On Thu, Jul 25, 2019 at 10:58:22AM -0400, Michael S. Tsirkin wrote: > On Thu, Jul 25, 2019 at 04:42:42PM +0200, Sergio Lopez wrote: > > > > Paolo Bonzini <pbonz...@redhat.com> writes: > > > > > On 25/07/19 15:26, Stefan Hajnoczi wrote: > > >> The microvm design has a premise and it can be answered definitively > > >> through performance analysis. > > >> > > >> If I had to explain to someone why PCI or ACPI significantly slows > > >> things down, I couldn't honestly do so. I say significantly because > > >> PCI init definitely requires more vmexits but can it be a small > > >> number? For ACPI I have no idea why it would consume significant > > >> amounts of time. > > > > > > My guess is that it's just a lot of code that has to run. :( > > > > I think I haven't shared any numbers about ACPI. > > > > I don't have details about where exactly the time is spent, but > > compiling a guest kernel without ACPI decreases the average boot time in > > ~12ms, and the kernel's unstripped ELF binary size goes down in a > > whooping ~300KiB. > > At least the binary size is hardly surprising. > > I'm guessing you built in lots of drivers. > > It would be educational to try to enable ACPI core but disable all > optional features.
Trying with ACPI_REDUCED_HARDWARE_ONLY would also be educational. > > > On the other hand, removing ACPI from QEMU decreases its initialization > > time in ~5ms, and the binary size is ~183KiB smaller. > > Yes - ACPI generation uses a ton of allocations and data copies. > > Need to play with pre-allocation strategies. Maybe something > as simple as: > > diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c > index f3fdfefcd5..24becc069e 100644 > --- a/hw/i386/acpi-build.c > +++ b/hw/i386/acpi-build.c > @@ -2629,8 +2629,10 @@ void acpi_build(AcpiBuildTables *tables, MachineState > *machine) > acpi_get_pci_holes(&pci_hole, &pci_hole64); > acpi_get_slic_oem(&slic_oem); > > +#define DEFAULT_ARRAY_SIZE 16 > table_offsets = g_array_new(false, true /* clear */, > - sizeof(uint32_t)); > + sizeof(uint32_t), > + DEFAULT_ARRAY_SIZE); > ACPI_BUILD_DPRINTF("init ACPI tables\n"); > > bios_linker_loader_alloc(tables->linker, > > will already help a bit. > > > > > IMHO, those are pretty relevant savings on both fronts. > > > > >> Until we have this knowledge, the premise of microvm is unproven and > > >> merging it would be premature because maybe we can get into the same > > >> ballpark by optimizing existing code. > > >> > > >> I'm sorry for being a pain. I actually think the analysis will > > >> support microvm, but it still needs to be done in order to justify it. > > > > > > No, you're not a pain, you're explaining your reasoning and that helps. > > > > > > To me *maintainability is the biggest consideration* when introducing a > > > new feature. "We can do just as well with q35" is a good reason to > > > deprecate and delete microvm, but not a good reason to reject it now as > > > long as microvm is good enough in terms of maintainability. Keeping it > > > out of tree only makes it harder to do this kind of experiment. virtio > > > 1 seems to be the biggest remaining blocker and I think it'd be a good > > > thing to have even for the ARM virt machine type. > > > > > > FWIW the "PCI tax" seems to be ~10 ms in QEMU, ~10 ms in the firmware(*) > > > and ~25 ms in the kernel. I must say that's pretty good, but it's still > > > 30% of the whole boot time and reducing it is the hardest part. If > > > having microvm in tree can help reducing it, good. Yes, it will get > > > users, but most likely they will have to support pc or q35 as a fallback > > > so we could still delete microvm at any time with the due deprecation > > > period if it turns out to be a failed experiment. > > > > > > Whether to use qboot or SeaBIOS for microvm is another story, but it's > > > an implementation detail as long as the ROM size doesn't change and/or > > > we don't do versioned machine types. So we can switch from one to the > > > other at any time; we can also include qboot directly in QEMU's tree, > > > without going through a submodule, which also reduces the infrastructure > > > needed (mirrors, etc.) and makes it easier to delete it. > > > > > > Paolo > > > > > > (*) I measured 15ms in SeaBIOS and 5ms in qboot from the first to the > > > last write to 0xcf8. I suspect part of qboot's 10ms boot time actually > > > end up measured as PCI in SeaBIOS, due to different init order, so the > > > real firmware cost of PAM and PCI initialization should be 5ms for qboot > > > and 10ms for SeaBIOS. > > > >