RE: [PATCH v3 1/3] x86/vmx: implement VMExit based guest Bus Lock detection

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Sent: Monday, July 4, 2022 6:07 PM
> 
> On Mon, Jul 04, 2022 at 11:27:37AM +0200, Jan Beulich wrote:
> > On 01.07.2022 15:16, Roger Pau Monne wrote:
> > > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > > @@ -4065,6 +4065,11 @@ void vmx_vmexit_handler(struct
> cpu_user_regs *regs)
> > >
> > >      if ( unlikely(exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY) )
> > >          return vmx_failed_vmentry(exit_reason, regs);
> > > +    if ( unlikely(exit_reason & VMX_EXIT_REASONS_BUS_LOCK) )
> > > +    {
> > > +        perfc_incr(buslock);
> > > +        exit_reason &= ~VMX_EXIT_REASONS_BUS_LOCK;
> > > +    }
> >
> > To cover for the flag bit, don't you also need to mask it off in
> > nvmx_idtv_handling()? Or (didn't go into detail with checking whether
> > there aren't any counter indications) pass the exit reason there from
> > vmx_vmexit_handler(), instead of re-reading it from the VMCS?
> 
> This seem to be an existing issue with nvmx_idtv_handling(), as it
> should use just the low 16bits to check against the VM Exit reason
> codes.
> 
> I can send a pre-patch to fix it, could pass exit reason from
> vmx_vmexit_handler(), but I would still need to cast to uint16_t for
> comparing against exit reason codes, as there's a jump into the 'out'
> label before VMX_EXIT_REASONS_BUS_LOCK is masked out.

or just masking out the bit in an earlier place which then also
covers nvmx_n2_vmexit_handler() below? There are a few other
goto's and return's before the point where that bit is currently
masked out. Having bus lock counted even in those failure paths
is also not a bad thing imho...

> 
> I think there's a similar issue with nvmx_n2_vmexit_handler() that
> doesn't cast the value to uint16_t and is called before
> VMX_EXIT_REASONS_BUS_LOCK is removed from exit reason.
>