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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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