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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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