linux-kernel - Re: [PATCH] KVM: x86: Add host physical address width capability

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Message-ID: <559FDDC7.3060306@redhat.com>
Date:	Fri, 10 Jul 2015 16:59:19 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Laszlo Ersek <lersek@...hat.com>, Bandan Das <bsd@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	qemu-devel@...gnu.org
Subject: Re: [PATCH] KVM: x86: Add host physical address width capability



On 10/07/2015 16:57, Laszlo Ersek wrote:
> > > ... In any case, please understand that I'm not campaigning for this
> > > warning :) IIRC the warning was your (very welcome!) idea after I
> > > reported the problem; I'm just trying to ensure that the warning match
> > > the exact issue I encountered.
> > 
> > Yup.  I think the right thing to do would be to hide memory above the
> > limit.
> How so?
> 
> - The stack would not be doing what the user asks for. Pass -m <a_lot>,
> and the guest would silently see less memory. If the user found out,
> he'd immediately ask (or set out debugging) why. I think if the user's
> request cannot be satisfied, the stack should fail hard.

That's another possibility.  I think both of them are wrong depending on
_why_ you're using "-m <a lot>" in the first place.

Considering that this really happens (on Xeons) only for 1TB+ guests,
it's probably just for debugging and then hiding the memory makes some
sense.

Paolo
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