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Re: [Qemu-devel] [PATCH 3/5] x86: fill high bits of mtrr mask

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 3/5] x86: fill high bits of mtrr mask
Date: Fri, 17 Jun 2016 17:29:21 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0

On 17/06/2016 17:27, Eduardo Habkost wrote:
> On Fri, Jun 17, 2016 at 04:25:57PM +0200, Paolo Bonzini wrote:
>> On 17/06/2016 15:41, Eduardo Habkost wrote:
>>> In theory, we should never initialize anything on the machine
>>> based on the host we are running. In practice we sometimes do
>>> that, and we know it's unsafe. Sending the value on the migration
>>> stream is a solution to detect when this breaks something. We did
>>> that before, for TSC frequency.
>>> I prefer to add a little extra code, than to waste time debugging
>>> when that stuff breaks.
>> Fair enough (but still let's add and strip the 1s in pre_save and
>> post_load).  It is a good justification for sending configuration over
>> the wire, that we initialize things based on the host we're running.
>> But we shouldn't initialize more things based on the host (e.g. new
>> booleans should be "enforce"-style).
> Strongly agreed.
>> Also I don't really like
>> introducing sanity checks for those that aren't based on the host (as in
>> the LMCE thread).  This code will never trigger in practice, so it's
>> just extra cost for no benefit.
> Our experience with guest ABI and compatibility code tells me
> otherwise. Keeping guest ABI is hard, so I would always support
> adding extra code to help us catch mistakes (either in QEMU,
> management software, or user mistakes).

I agree, but I think the "enforce"-style properties are enough (and
count as extra code :)).


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