Zero-copy block driver?

Zero-copy block driver?

[Darko Petrović]

Hi everyone,

Could you please tell me if it is possible to use a block driver that 
completely avoids the guest kernel and copies block data directly 
to/from the given buffer in the guest userspace?
If yes, how to activate it? If not... why not? :)

Thanks!
Darko

Re: Zero-copy block driver?

[Stefan Hajnoczi]

2011/1/29 Darko Petrović <darko.b.petrovic@gmail.com>:
> Could you please tell me if it is possible to use a block driver that
> completely avoids the guest kernel and copies block data directly to/from
> the given buffer in the guest userspace?
> If yes, how to activate it? If not... why not? :)

Inside the guest, open files using the O_DIRECT flag.  This tells the
guest kernel to avoid the page cache when possible, enabling
zero-copy.  You need to use aligned memory buffers and perform I/O in
multiples of the block size.

See the open(2) man page for details.  Make sure you really want to do
this, most applications don't.

Stefan

Re: Zero-copy block driver?

[Darko Petrović]

Thanks for your help. Actually, I am more interested in doing it from 
the outside, if possible (I am not allowed to change the application 
code). Can the guest be tricked by KVM somehow, using the appropriate 
drivers? Just to clear it out, copying to/from a host buffer is fine, I 
just want to avoid having guest buffers.

Cheers!
Darko

On 01/29/2011 10:49 AM, Stefan Hajnoczi wrote:
> Inside the guest, open files using the O_DIRECT flag. This tells the
> guest kernel to avoid the page cache when possible, enabling
> zero-copy.  You need to use aligned memory buffers and perform I/O in
> multiples of the block size.
>
> See the open(2) man page for details.  Make sure you really want to do
> this, most applications don't.
>
> Stefan

Re: Zero-copy block driver?

[Stefan Hajnoczi]

2011/1/29 Darko Petrović <darko.b.petrovic@gmail.com>:
> Thanks for your help. Actually, I am more interested in doing it from the
> outside, if possible (I am not allowed to change the application code). Can
> the guest be tricked by KVM somehow, using the appropriate drivers? Just to
> clear it out, copying to/from a host buffer is fine, I just want to avoid
> having guest buffers.

Not really.  If the application is designed to use the page cache then
it will use it.

You might want to look at unmapped page cache control which is not in
mainline Linux yet:
http://lwn.net/Articles/419713/

Stefan