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

On Tue, Feb 17, 2015 at 4:32 AM, Michael S. Tsirkin [off-list ref] wrote:

On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote:

quoted

On 17/02/2015 10:02, Michael S. Tsirkin wrote:

quoted

quoted

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 <redacted>

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.

quoted

quoted

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 majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html