Re: [PATCH] vhost: support upto 509 memory regions

[Michael S. Tsirkin]

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
> > --
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