Re: [Xen-devel] Re: [GIT PULL] xen /proc/mtrr implementation

[Ingo Molnar]

* Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:

> On 05/19/09 15:31, Ingo Molnar wrote:
>> * Gerd Hoffmann<kraxel@xxxxxxxxxx>  wrote:
>>
>>> On 05/19/09 14:26, Ingo Molnar wrote:
>>>> * Gerd Hoffmann<kraxel@xxxxxxxxxx>   wrote:
>>>>
>>>>> On 05/19/09 13:08, Ingo Molnar wrote:
>>>>>> Or, alternatively, the hypervisor can expose its own administrative
>>>>>> interface to manage MTRRs.
>>>>> Guess what?  Xen does exactly that.  And the xen mtrr_ops
>>>>> implementation uses that interface ...
>>>> No, that is not an 'administrative interface' - that is a guest
>>>> kernel level hack that complicates Linux, extends its effective ABI
>>>> dependencies and which has to be maintained there from that point
>>>> on.
>>>>
>>>> There's really just three proper technical solutions here:
>>>>
>>>> - either catch the lowlevel CPU hw ops (the MSR modifications, which
>>>>     isnt really all that different from the mtrr_ops approach so it
>>>>     shouldnt pose undue difficulties to the Xen hypervisor).
>>> Devil is in the details.
>>>
>>> The dom0 kernel might not see all physical cpus on the system.  So
>>> Xen can't leave the job of looping over all cpus to the dom0
>>> kernel, Xen has to apply the changes made by the (priviledged)
>>> guest kernel on any (virtual) cpu to all (physical) cpus in the
>>> machine.
>>
>> Applying MTRR changes to only part of the CPUs is utter madness.
>
> Sure.  Do you read what I'm writing?
>
>>> Which in turn means the "lowlevel cpu hw op" would work in a
>>> slightly different way on Xen and native.  Nasty.
>>>
>>>>     That will
>>>>     be maximally transparent and compatible, with zero changes needed
>>>>     to the Linux kernel.
>>> No, the linux kernel probably should do the wrmsr on one cpu only then.
>>
>> Why?
>
> See above.  Xen has to apply the changes to all cpus anyway.

do _you_ read what i wrote, in the thread you are replying to:

|
| The change of MTRR's on _any_ of the guest CPUs in a dom0 context 
| should immediately be refected on all CPUs. Assymetric MTRR 
| settings are madness.
|


>>> Oops, the third "proper technical solutions" is missing.
>>
>> Yeah, the third one is to not touch MTRRs after bootup and use PAT.
>
> Works only in case the CPU has PAT support.

Which specific CPU without PAT support do you worry about?

	Ingo
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