RE: [Xen-devel][PATCH 02/02] VMX: Add HVM RDTSCP support

["Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>]

Hi, Jan, 
        There are two kinds of conditions for adding new MSRs, let's call it 
CONDITION 1
and CONDITION 2.
1. Developer adding new MSR both in msr_index[] and the enum.
2. Developer adding new MSR *ONLY* in enum. (Like TSC_AUX MSR)

My original thought is that, since both msr_index and enum is seldom modified, 
so I 
add that BUILD_BUG_ON(MSR_INDEX_SIZE != 3) to reminder the developer that 
currently
we only save three MSRs for host state. However, as you said, he need to adjust 
the 
BUILD_BUG_ON Condition each time for "CONDITION 1". But if he only adds MSR into
enum (CONDITION 2, also like the TSC_AUX MSR), then he is not needed to modify 
the
BUILD_BUG_ON Condition.

I understand what you mean now. This is to reminder developer that, if he want 
to add
new MSR both into the enum and msr_index[], it should be placed in the end of 
msr_index[],
but BEFORE VMX_INDEX_MSR_TSC_AUX in enum. Then he is not needed to modify he
following condition check.
"ARRAY_SIZE(msr_index) == VMX_INDEX_MSR_TSC_AUX && 
VMX_INDEX_MSR_TSC_AUX == VMX_MSR_COUNT - 1".
But it he ONLY wants to add the MSR in enum (CONDITION 2, also like the TSC_AUX 
MSR),
the modifcation to BUILD_BUG_ON condition is needed:
"ARRAY_SIZE(msr_index) == VMX_INDEX_MSR_TSC_AUX && 
VMX_INDEX_MSR_TSC_AUX == VMX_MSR_COUNT - 2".

Both approaches have the effect to reminder that, MSRs in enum is not the same 
as MSRs
in msr_index[]. Anyway I attached the patch according to your suggestion.

Jan Beulich wrote:
>>>> "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx> 11.12.09 17:22 >>>
>>      Here is the updated one, I add the BUILD_BUG_ON() and some comments.
> 
> I still don't think that's what we want: It's not a bug if
> MSR_INDEX_SIZE is not 3. A BUILD_BUG_ON() should check things that
> ought to be guaranteed, but shouldn't require adjustment each time
> some table 
> gets expanded/shrunk. What you want to minimally check is
> ARRAY_SIZE(msr_index) == VMX_INDEX_MSR_TSC_AUX &&
> VMX_INDEX_MSR_TSC_AUX == VMX_MSR_COUNT - 1.
> 
> Jan

Attachment: 02-add-hvm-rdtscp-support.patch
Description: 02-add-hvm-rdtscp-support.patch

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