Re: [kvm-unit-tests PATCH] x86: nvmx: test max atomic switch MSRs

[Marc Orr <marcorr@google.com>]

> > +static void *alloc_2m_page(void)
> > +{
> > +     return alloc_pages(PAGE_2M_ORDER);
> > +}
>
> Allocating 2mb pages is complete overkill.  The absolute theoretical max
> for the number of MSRs is (8 * 512) = 4096, for a total of 32kb per list.
> We can even show the math so that it's obvious how the size is calculated.
> Plus one order so we can test overrun.
> /*
>  * The max number of MSRs is specified in 3 bits bits, plus 1. I.e. 7+1==8.
>  * Allocate 64k bytes of data to cover max_msr_list_size and then some.
>  */
> static const u32 msr_list_page_order = 4;

8 * 512 should be 16 * 512, right [1]? Therefore, the maximum
theoretical is 64 kB.

I'll happily apply what you've suggested in v2. But I don't see why
it's so terrible to over-allocate here. Leveraging a generic 2 MB page
allocator can be reused going forward, and encourages uniformity
across tests.

[1]
struct vmx_msr_entry {
  u32 index;
  u32 reserved;
  u64 value;
} __aligned(16);

> > +static void populate_msr_list(struct vmx_msr_entry *msr_list, int count)
> > +{
> > +     int i;
> > +
> > +     for (i = 0; i < count; i++) {
> > +             msr_list[i].index = MSR_IA32_TSC;
> > +             msr_list[i].reserved = 0;
> > +             msr_list[i].value = 0x1234567890abcdef;
>
> Maybe overkill, but we can use a fast string op for this.  I think
> I got the union right?
>
> static void populate_msr_list(struct vmx_msr_entry *msr_list, int count)
> {
>         union {
>                 struct vmx_msr_entry msr;
>                 u64 val;
>         } tmp;
>
>         tmp.msr.index = MSR_IA32_TSC;
>         tmp.msr.reserved = 0;
>         tmp.msr.value = 0x1234567890abcdef;
>
>         asm volatile (
>                 "rep stosq\n\t"
>                 : "=c"(count), "=D"(msr_list)
>                 : "a"(tmp.val), "c"(count), "D"(msr_list)
>                 : "memory"
>         );
> }

:-). I do think it's overkill. However, I'm OK to apply this
suggestion in v2 if everyone is OK with me adding a comment that
explains it in terms of the original code, so that x86 noobs, like
myself, can understand what's going on.

> This is a misleading name, e.g. it took me quite a while to realize this
> is testing only the passing scenario.  For me, "limit test" implies that
> it'd be deliberately exceeding the limit, or at least testing both the
> passing and failing cases.  I suppose we can't easily test the VMX abort
> cases, but we can at least test VM_ENTER_LOAD.
>
> Distilling things down to the bare minimum yields something like the
> following.

Looks excellent overall. Still not clear what the consensus is on
whether or not to test the VM-entry failure. I think a flag seems like
a reasonable compromise. I've never added a flag to a kvm-unit-test,
so I'll see if I can figure that out.