linux-kernel - Re: [PATCH 1/2] vhost: add ioctl to query nregions upper limit

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Date:	Wed, 29 Jul 2015 18:23:05 +0300
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
Subject: Re: [PATCH 1/2] vhost: add ioctl to query nregions upper limit

On Wed, Jul 29, 2015 at 05:01:43PM +0200, Paolo Bonzini wrote:
> 
> 
> On 29/07/2015 16:56, Michael S. Tsirkin wrote:
> >>>> > > > Also, document our contract with legacy userspace: when running on an
> >>>> > > > old kernel, you get -1 and you can assume at least 64 slots.  Since 0
> >>>> > > > value's left unused, let's make that mean that the current userspace
> >>>> > > > behaviour (trial and error) is required, just in case we want it back.
> >>> > > 
> >>> > > What's wrong with reading the module parameter value? It's there in
> >>> > > sysfs ...
> >> > for most cases it would work but distro doesn't have to mount
> >> > sysfs under /sys
> > If it wants to rewrite all userspace, sure it doesn't.
> 
> I agree, on the other hand it doesn't seem far fetched to have a per-fd
> maximum in the future.  So I think this patch is more future-proof.
> 
> Paolo

Possibly, but this calls for some kind of priveledge separation model,
where a priveledged app can set the per-fd limit while regular ones
can only read it. Sounds very complex. Let's see some of this code
first.

And I really think there are better ways to future proof it, e.g. teach
userspace to do error handling, revert adding a slot if one of the
components can't support it.

-- 
MST
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