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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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).

Jan


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