Re: [PATCH v3 2/6] KVM: x86: add force_intercept_exceptions_mask

[Sean Christopherson <seanjc@google.com>]

Assuming this hasn't been abandoned...

On Wed, Aug 11, 2021, Maxim Levitsky wrote:
> This parameter will be used by VMX and SVM code to force
> interception of a set of exceptions, given by a bitmask
> for guest debug and/or kvm debug.
> 
> This is based on an idea first shown here:
> https://patchwork.kernel.org/project/kvm/patch/20160301192822.GD22677@pd.tnic/
> 
> CC: Borislav Petkov <bp@suse.de>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/kvm/x86.c | 3 +++
>  arch/x86/kvm/x86.h | 2 ++
>  2 files changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fdc0c18339fb..092e2fad3c0d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -184,6 +184,9 @@ module_param(force_emulation_prefix, bool, S_IRUGO);
>  int __read_mostly pi_inject_timer = -1;
>  module_param(pi_inject_timer, bint, S_IRUGO | S_IWUSR);
>  
> +uint force_intercept_exceptions_mask;
> +module_param(force_intercept_exceptions_mask, uint, S_IRUGO | S_IWUSR);

Use octal permissions.  This also can't be a simple writable param, at least not
without a well-documented disclaimer, as there's no guarantee a vCPU will update
its exception bitmap in a timely fashion.  An alternative to a module param would
be to extend/add a per-VM ioctl(), e.g. maybe KVM_SET_GUEST_DEBUG?  The downside
of an ioctl() is that it would require userspace enabling :-/