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

Maxim Levitsky
2022-02-08T14:35:15Z

On Thu, 2021-09-02 at 16:56 +0000, Sean Christopherson wrote:
> 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 :-/
>

All other module params in this file use macros for permissions, that is why
I used them too.

I'll add a comment with a disclaimer here - this is only for debug.
I strongly don't want to have this as ioctl as that will indeed need qemu patches,
not to mention things like unit tests and which don't even always use qemu.

Or I can make this parameter read-only. I don't mind reloading kvm module when
I change this parameter.

Best regards,
Maxim Levitsky

PS: Forgot to send this mail, it was in my drafts folder.