On 20/06/17 11:02, David Gibson wrote: > On Mon, Jun 19, 2017 at 12:32:48PM +0200, Thomas Huth wrote: >> On 19.06.2017 11:39, Alexander Graf wrote: >>> On 06/15/2017 03:33 PM, Peter Maydell wrote: >>>> I've just noticed that on a SPARC host, some of the PPC guests >>>> warn during make check: >>>> >>>> /ppc64/prom-env/pseries: >>>> qemu-system-ppc64: System page size 0x2000 is not enabled in >>>> page_size_mask (0x11000). Performance may be slow >>>> >>>> Is this really a performance problem on a TCG guest? >>>> It makes the 'make check' output a bit noisy, but not >>>> unbearably so. >>> >>> From what I can tell this really only affects DDW which should be page >>> size agnostic when only using emulated devices. Maybe Alexey thought of >>> using VFIO-pci devices in a TCG guest? >> >> Maybe we could at least silence the warning by checking >> qtest_enabled()? > > > So, > > 1) Yes, we should at least silence the warning.
Agree, wrapping it into kvm_enabled() should do it. > > 2) Certainly the mentioned slowdown shouldn't be significant for TCG > guests. It is not significant at all. While in-kernel TCE handling improves throughput for VFIO devices quite a lot (200mb/s -> 1000mb/s), TCG could not do any better than 20mb/s anyway. > 3) I _think_ the restriction which causes the slowdown doesn't even > exist for TCG, but I haven't 100% convinced myself of this. I've been > sick lately, so my brain isn't working all that well. Mismatch between system page size and IOMMU page size means H_PUT_TCE is going to be called more often than needed; if VFIO is involved, then more ioctl(vfio_container_fd, DMAP_MAP) calls. But - we cannot do 8K IOMMU pages at all, SPARC does not support VFIO (or does it?), and TCG is slow anyway. -- Alexey
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