Re: [PATCH v2 3/3] x86/vmx: implement Notify VM Exit

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Jun 07, 2022 at 09:43:25AM +0200, Jan Beulich wrote:
> On 03.06.2022 16:46, Roger Pau Monné wrote:
> > On Fri, Jun 03, 2022 at 02:49:54PM +0200, Jan Beulich wrote:
> >> On 26.05.2022 13:11, Roger Pau Monne wrote:
> >>> --- a/xen/arch/x86/hvm/vmx/vmx.c
> >>> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> >>> @@ -1419,10 +1419,19 @@ static void cf_check vmx_update_host_cr3(struct 
> >>> vcpu *v)
> >>>  
> >>>  void vmx_update_debug_state(struct vcpu *v)
> >>>  {
> >>> +    unsigned int mask = 1u << TRAP_int3;
> >>> +
> >>> +    if ( !cpu_has_monitor_trap_flag && cpu_has_vmx_notify_vm_exiting )
> >>
> >> I'm puzzled by the lack of symmetry between this and ...
> >>
> >>> +        /*
> >>> +         * Only allow toggling TRAP_debug if notify VM exit is enabled, 
> >>> as
> >>> +         * unconditionally setting TRAP_debug is part of the XSA-156 fix.
> >>> +         */
> >>> +        mask |= 1u << TRAP_debug;
> >>> +
> >>>      if ( v->arch.hvm.debug_state_latch )
> >>> -        v->arch.hvm.vmx.exception_bitmap |= 1U << TRAP_int3;
> >>> +        v->arch.hvm.vmx.exception_bitmap |= mask;
> >>>      else
> >>> -        v->arch.hvm.vmx.exception_bitmap &= ~(1U << TRAP_int3);
> >>> +        v->arch.hvm.vmx.exception_bitmap &= ~mask;
> >>>  
> >>>      vmx_vmcs_enter(v);
> >>>      vmx_update_exception_bitmap(v);
> >>> @@ -4155,6 +4164,9 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
> >>>          switch ( vector )
> >>>          {
> >>>          case TRAP_debug:
> >>> +            if ( cpu_has_monitor_trap_flag && 
> >>> cpu_has_vmx_notify_vm_exiting )
> >>> +                goto exit_and_crash;
> >>
> >> ... this condition. Shouldn't one be the inverse of the other (and
> >> then it's the one down here which wants adjusting)?
> > 
> > The condition in vmx_update_debug_state() sets the mask so that
> > TRAP_debug will only be added or removed from the bitmap if
> > !cpu_has_monitor_trap_flag && cpu_has_vmx_notify_vm_exiting (note that
> > otherwise TRAP_debug is unconditionally set if
> > !cpu_has_vmx_notify_vm_exiting).
> > 
> > Hence it's impossible to get a VMExit TRAP_debug with
> > cpu_has_monitor_trap_flag && cpu_has_vmx_notify_vm_exiting because
> > TRAP_debug will never be set by vmx_update_debug_state() in that
> > case.
> 
> Hmm, yes, I've been misguided by you not altering the existing setting
> of v->arch.hvm.vmx.exception_bitmap in construct_vmcs(). Instead you
> add an entirely new block of code near the bottom of the function. Is
> there any chance you could move up that adjustment, perhaps along the
> lines of
> 
>     v->arch.hvm.vmx.exception_bitmap = HVM_TRAP_MASK
>               | (paging_mode_hap(d) ? 0 : (1U << TRAP_page_fault))
>               | (v->arch.fully_eager_fpu ? 0 : (1U << TRAP_no_device));
>     if ( cpu_has_vmx_notify_vm_exiting )
>     {
>         __vmwrite(NOTIFY_WINDOW, vm_notify_window);
>         /*
>          * Disable #AC and #DB interception: by using VM Notify Xen is
>          * guaranteed to get a VM exit even if the guest manages to lock the
>          * CPU.
>          */
>         v->arch.hvm.vmx.exception_bitmap &= ~((1U << TRAP_debug) |
>                                               (1U << TRAP_alignment_check));
>     }
>     vmx_update_exception_bitmap(v);

Sure, will move up when posting a new version then.  I will wait for
feedback from Jun or Kevin regarding the default window size before
doing so.

Thanks, Roger.