Re: [Xen-devel] [PATCH 2/3] VMX: allocate VMCS pages from domain heap

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 24/11/15 11:04, Jan Beulich wrote:
>>>> On 24.11.15 at 11:59, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 24/11/15 10:55, Andrew Cooper wrote:
>>> On 24/11/15 07:50, Jan Beulich wrote:
>>>>>>> On 24.11.15 at 06:04, <kevin.tian@xxxxxxxxx> wrote:
>>>>>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>>>>>> Sent: Monday, November 23, 2015 10:28 PM
>>>>>>
>>>>>>>>> On 21.10.15 at 05:16, <kevin.tian@xxxxxxxxx> wrote:
>>>>>>>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>>>>>>>> Sent: Tuesday, October 20, 2015 6:36 PM
>>>>>>>>>>> On 20.10.15 at 12:12, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>>>>>> On 19/10/15 16:22, Jan Beulich wrote:
>>>>>>>>>> @@ -580,7 +583,7 @@ int vmx_cpu_up_prepare(unsigned int cpu)
>>>>>>>>>>  void vmx_cpu_dead(unsigned int cpu)
>>>>>>>>>>  {
>>>>>>>>>>      vmx_free_vmcs(per_cpu(vmxon_region, cpu));
>>>>>>>>>> -    per_cpu(vmxon_region, cpu) = NULL;
>>>>>>>>>> +    per_cpu(vmxon_region, cpu) = 0;
>>>>>>>>> While this is currently safe (as pa 0 is not part of the available 
>>>>>>>>> heap
>>>>>>>>> allocation range), might it be worth introducing a named sentential?  
>>>>>>>>> I
>>>>>>>>> can forsee a DMLite nested Xen scenario where we definitely don't need
>>>>>>>>> to treat the low 1MB magically.
>>>>>>>> I guess there are more things to adjust if we ever cared to recover
>>>>>>>> the few hundred kb below 1Mb. And then I don't see why nested
>>>>>>>> Xen would matter here: One major main reason for reserving that
>>>>>>>> space is that we want to put the trampoline there. Do you think
>>>>>>>> DMlite would allow us to get away without? But even if so, this
>>>>>>>> would again fall under what I've said in the first sentence.
>>>>>>> Could you at least introduce a macro first? Regardless of how much
>>>>>>> things to adjust, this way makes future change simple.
>>>>>> So I've made an attempt, but this is really getting unwieldy: Setting
>>>>>> per-CPU data to non-zero initial values is not possible; making sure
>>>>>> cleanup code avoids assuming such variables got initialized is quite
>>>>>> error prone. Same goes at least to a certain extent for struct vcpu
>>>>>> members (see e.g. nvmx_vcpu_destroy(), which currently is
>>>>>> correct no matter whether nvmx_vcpu_initialise() ran at all, or to
>>>>>> completion).
>>>>>>
>>>>>> I also don't see what a macro would help here, or how/where it
>>>>>> should be used. paddr_valid()? Yes, I could do this, but it wouldn't
>>>>>> simplify much when later wanting to convert to a non-zero value
>>>>>> for above reasons (it would instead give the wrong impression that
>>>>>> changing the value is all it takes).
>>>>>>
>>>>> Thanks for looking into this attempt. Based on your explanation
>>>>> I think your original code is reasonable to go. Here is my ack:
>>>>>
>>>>> Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx>
>>>> Thanks Kevin. Andrew - please indicate whether your previous
>>>> comment is to be considered a NAK, or "just a comment".
>>> I would prefer a sentinel value being introduced, but can live without
>>> it being changed.  It is definitely not the only area which uses 0 as a
>>> sentinel and cleanup will have to happen, one way or another.
>> Actually it turns out that we already have an appropriate sentinel.
>>
>> include/asm-x86/types.h:34:#define INVALID_PADDR (~0UL)
> Yes, that's what I had tried to use in above mentioned attempt.

Right.  Lets not block the change on another infrastructure improvement.

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

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