Re: [Qemu-devel] [PATCH v2] pc: memhp: enforce minimal 128Mb alignment for pc-dimm

[Eduardo Habkost]

On Tue, Oct 27, 2015 at 10:14:56AM +0100, Igor Mammedov wrote:
> On Tue, 27 Oct 2015 10:53:08 +0200
> "Michael S. Tsirkin" <address@hidden> wrote:
> 
> > On Tue, Oct 27, 2015 at 09:48:37AM +0100, Igor Mammedov wrote:
> > > On Tue, 27 Oct 2015 10:31:21 +0200
> > > "Michael S. Tsirkin" <address@hidden> wrote:
> > > 
> > > > On Mon, Oct 26, 2015 at 02:24:32PM +0100, Igor Mammedov wrote:
> > > > > Yep it's workaround but it works around QEMU's broken virtio
> > > > > implementation in a simple way without need for guest side changes.
> > > > > 
> > > > > Without foreseeable virtio fix it makes memory hotplug unusable and 
> > > > > even
> > > > > more so if there were a virtio fix it won't fix old guests since 
> > > > > you've
> > > > > said that virtio fix would require changes of both QEMU and guest 
> > > > > sides.
> > > > 
> > > > What makes it not foreseeable?
> > > > Apparently only the fact that we have a work-around in place so no one
> > > > works on it.  I can code it up pretty quickly, but I'm flat out of time
> > > > for testing as I'm going on vacation soon, and hard freeze is pretty
> > > > close.
> > > I can lend a hand for testing part.
> > > 
> > > > 
> > > > GPA space is kind of cheap, but wasting it in chunks of 512M
> > > > seems way too aggressive.
> > > hotplug region is sized with 1Gb alignment reserve per DIMM so we aren't
> > > actually wasting anything here.
> > >
> > 
> > If I allocate two 1G DIMMs, what will be the gap size? 512M? 1G?
> > It's too much either way.
> minimum would be 512, and if backend is 1Gb-hugepage gap will be
> backend's natural alignment (i.e. 1Gb).

Is backend configuration even allowed to affect the machine ABI? We need
to be able to change backend configuration when migrating the VM to
another host.

-- 
Eduardo