linux-kernel - Re: [PATCH 3/5] vhost: support upto 509 memory regions

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Date:	Fri, 19 Jun 2015 18:33:39 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Igor Mammedov <imammedo@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, andrey@...l.ru
Subject: Re: [PATCH 3/5] vhost: support upto 509 memory regions

On Fri, Jun 19, 2015 at 06:26:27PM +0200, Paolo Bonzini wrote:
> 
> 
> On 19/06/2015 18:20, Michael S. Tsirkin wrote:
> > > We could, but I/O is just an example.  It can be I/O, a network ring,
> > > whatever.  We cannot audit all address_space_map uses.
> > > 
> >
> > No need to audit them all: defer device_add using an hva range until
> > address_space_unmap drops using hvas in range drops reference count to
> > 0.
> 
> That could be forever.  You certainly don't want to lockup the monitor
> forever just because a device model isn't too friendly to memory hot-unplug.

We can defer the addition, no need to lockup the monitor.

> That's why you need to audit them (also, it's perfectly in the device
> model's right to use address_space_unmap this way: it's the guest that's
> buggy and leaves a dangling reference to a region before unplugging it).
> 
> Paolo

Then maybe it's not too bad that the guest will crash because the memory
was unmapped.

-- 
MST
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