On Tue, Feb 17, 2015 at 04:53:45PM -0800, Eric Northup wrote: > On Tue, Feb 17, 2015 at 4:32 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> 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 <imammedo@xxxxxxxxxx> > >> > > >> > 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. AFAIK this is pretty much a requirement for both kvm and vhost, as we require each guest page to also be mapped in qemu memory. > > 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 majordomo@xxxxxxxxxxxxxxx > > 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 majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
