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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> 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

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