Re: [Qemu-devel] [PATCH V11 2/3] i386: Add a Virtual Machine Generation ID device

[Michael S. Tsirkin]

On Wed, Feb 04, 2015 at 04:53:54PM +0100, Igor Mammedov wrote:
> > > > Isn't this will cause a VMEXIT when the guest is reading the GUID? If it
> > > > is then this idea was already presented and Michael didn't approve it.
> > > It will, but is it performance critical? VM supposed to read it
> > > at start-up and on getting notification. So I think VMEXIT in this case
> > > is not sufficient to drop simple and strait-forward design.
> > 
> > I agree with you on that and one of the previous patches did used a 
> > fixed-address to store the GUID while read/write access were handled by 
> > qemu driver code. But as I wrote before, it was Michael who didn't approved 
> > it so I proposed this method although it is a bit more complicated.
> > 
> > I don't know how to break out of this dead-lock... :(
> Could you post a link to driver based version of series.
> Perhaps we could address Michael's comments and still stay
> with a simple implementation.

The point is to keep all allocations in guest.
I don't want to "steal" a page from guest.

> > 
> > > 
> > > BTW:
> > > For start-up fw_cfg file is not any way better, it's also causes VMEXIT
> > > for every byte it reads from it.
> > 
> > I don't understand your claim. Accessing the fw_cfg "file" doesn't cause 
> > VMEXIT as it located somewhere in the guest's memory range.
> As far as I'm aware MMIO or ioport is used for reading fw_cfg contents
> on guest side, one byte at a time and every such access causes VMEXIT
> into QEMU callback.

It's highly unlikely to be measureable. Prove me wrong if you like.

-- 
MST