Re: [kvm-unit-tests PATCH 1/2] x86: access: Fix timeout failure by limiting number of flag combinations

[Sean Christopherson <seanjc@google.com>]

On Wed, Aug 11, 2021, Babu Moger wrote:
> 
> On 8/11/21 2:09 AM, Paolo Bonzini wrote:
> > On 11/08/21 01:38, Babu Moger wrote:
> >> No. This will not work. The PKU feature flag is bit 30. That is 2^30
> >> iterations to cover the tests for this feature. Looks like I need to split
> >> the tests into PKU and non PKU tests. For PKU tests I may need to change
> >> the bump frequency (in ac_test_bump_one) to much higher value. Right now,
> >> it is 1. Let me try that,
> > 
> > The simplest way to cut on tests, which is actually similar to this patch,
> > would be:
> > 
> > - do not try all combinations of PTE access bits when reserved bits are set
> > 
> > - do not try combinations with more than one reserved bit set
> 
> Did you mean this? Just doing this reduces the combination by huge number.
> I don't need to add your first PTE access combinations.
> 
> diff --git a/x86/access.c b/x86/access.c
> index 47807cc..a730b6b 100644
> --- a/x86/access.c
> +++ b/x86/access.c
> @@ -317,9 +317,7 @@ static _Bool ac_test_legal(ac_test_t *at)
>      /*
>       * Shorten the test by avoiding testing too many reserved bit
> combinations
>       */
> -    if ((F(AC_PDE_BIT51) + F(AC_PDE_BIT36) + F(AC_PDE_BIT13)) > 1)
> -        return false;
> -    if ((F(AC_PTE_BIT51) + F(AC_PTE_BIT36)) > 1)
> +    if ((F(AC_PDE_BIT51) + F(AC_PDE_BIT36) + F(AC_PDE_BIT13) +
> F(AC_PTE_BIT51) + F(AC_PTE_BIT36)) > 1)
>          return false;
> 
>      return true;

Looks good to me, is it sufficient to keep the overall runtime sane?.  And maybe
update the comment too, e.g. something like

	/*
	 * Skip testing multiple reserved bits to shorten the test.  Reserved
	 * bit page faults are terminal and multiple reserved bits do not affect
	 * the error code; the odds of a KVM bug are super low, and the odds of
	 * actually being able to detect a bug are even lower.
	 */