On Tue, Feb 17, 2015 at 4:32 AM, Michael S. Tsirkin <m...@redhat.com> wrote: > On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: >> >> >> On 17/02/2015 10:02, Michael S. Tsirkin wrote: >> > > Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 >> > > to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. >> > > >> > > Signed-off-by: Igor Mammedov <imamm...@redhat.com> >> > >> > This scares me a bit: each region is 32byte, we are talking >> > a 16K allocation that userspace can trigger. >> >> What's bad with a 16K allocation? > > It fails when memory is fragmented. > >> > How does kvm handle this issue? >> >> It doesn't. >> >> Paolo > > I'm guessing kvm doesn't do memory scans on data path, > vhost does. > > qemu is just doing things that kernel didn't expect it to need. > > Instead, I suggest reducing number of GPA<->HVA mappings: > > you have GPA 1,5,7 > map them at HVA 11,15,17 > then you can have 1 slot: 1->11 > > To avoid libc reusing the memory holes, reserve them with MAP_NORESERVE > or something like this.
This works beautifully when host virtual address bits are more plentiful than guest physical address bits. Not all architectures have that property, though. > We can discuss smarter lookup algorithms but I'd rather > userspace didn't do things that we then have to > work around in kernel. > > > -- > MST > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
