Re: [PATCH] kvm: x86: Make traced and returned value of kvm_cpuid consistent again

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, Mar 02, 2020 at 05:11:57PM +0100, Paolo Bonzini wrote:
> Queued, thanks.

Too fast, too fast!

On Sun, Mar 01, 2020 at 11:47:20AM +0100, Jan Kiszka wrote:
> From: Jan Kiszka <jan.kiszka@siemens.com>
>
> After 43561123ab37, found is not set correctly in case of leaves 0BH,
> 1FH, or anything out-of-range.

No, found is set correctly, kvm_cpuid() should return true if and only if
an exact match for the requested function is found, and that's the original
tracing behavior of "found" (pre-43561123ab37).

> This is currently harmless for the return value because the only caller
> evaluating it passes leaf 0x80000008.

No, it's 100% correct.  Well, technically it's irrelevant because the only
caller, check_cr_write(), passes %false for check_limit, i.e. found will be
true if and only if entry 0x80000008 exists.  But, in a purely hypothetical
scenario where the emulator passed check_limit=%true, the intent of "found"
is to report that the exact leaf was found, not if some random entry was
found.

> However, the trace entry is now misleading due to this inaccuracy. It is
> furthermore misleading because it reports the effective function, not
> the originally passed one. Fix that as well.
>
> Fixes: 43561123ab37 ("kvm: x86: Improve emulation of CPUID leaves 0BH and 1FH")
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
>  arch/x86/kvm/cpuid.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index b1c469446b07..79a738f313f8 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1000,13 +1000,12 @@ static bool cpuid_function_in_range(struct kvm_vcpu *vcpu, u32 function)
>  bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>              u32 *ecx, u32 *edx, bool check_limit)
>  {
> -     u32 function = *eax, index = *ecx;
> +     u32 orig_function = *eax, function = *eax, index = *ecx;
>       struct kvm_cpuid_entry2 *entry;
>       struct kvm_cpuid_entry2 *max;

Rather than add another variable, this can be cleaned up to remove "max".
cpuid_function_in_range() also has a bug.  I've got patches, in the process
of whipping up a unit test.