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

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Date:	Fri, 19 Jun 2015 18:44:44 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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 19/06/2015 18:33, Michael S. Tsirkin wrote:
> 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.

Patches are welcome.

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

That's a matter of taste.  I strongly prefer using 12K extra memory per
VCPU to a guest crash.

Paolo
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