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

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Message-ID: <559FDF8F.1020109@redhat.com>
Date:	Fri, 10 Jul 2015 17:06:55 +0200
From:	Laszlo Ersek <lersek@...hat.com>
To:	Paolo Bonzini <pbonzini@...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 07/10/15 16:59, Paolo Bonzini wrote:
> 
> 
> 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,

I reported this issue because I ran into it with a ~64GB guest. From my
/proc/cpuinfo:

model name      : Intel(R) Core(TM) i7 CPU       M 620  @ 2.67GHz
address sizes   : 36 bits physical, 48 bits virtual

I was specifically developing 64GB+ support for OVMF, and this
limitation caused me to think that there was a bug in my OVMF patches.
(There wasn't.) An error message from QEMU, advising me to turn off EPT,
would have saved me many hours.

Thanks
Laszlo

> it's probably just for debugging and then hiding the memory makes some
> sense.
> 
> Paolo
> 

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