Re: [PATCH] x86: Add Kconfig option for log-dirty tracking

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

On Mon Feb 9, 2026 at 4:55 PM CET, Jan Beulich wrote:
> On 09.02.2026 16:24, Alejandro Vallejo wrote:
>> On Mon Feb 9, 2026 at 3:48 PM CET, Jan Beulich wrote:
>>> On 09.02.2026 11:31, Alejandro Vallejo wrote:
>>>> --- a/xen/arch/x86/Kconfig
>>>> +++ b/xen/arch/x86/Kconfig
>>>> @@ -146,6 +146,7 @@ config XEN_IBT
>>>>  config SHADOW_PAGING
>>>>    bool "Shadow Paging"
>>>>    default !PV_SHIM_EXCLUSIVE
>>>> +  select LOG_DIRTY
>>>>    depends on PV || HVM
>>>>    help
>>>
>>> Why would this be? IOW why would shadow imply log-dirty, but HAP wouldn't?
>> 
>> The logic is rather opaque. I admit I'm a bit fuzzy on the uses of logdirty.
>> 
>> I know what it's for and I could navigate the code if a problem arose, but 
>> I'm
>> less clear about which other elements of the hypervisor rely on it (pod? 
>> nsvm?
>> vvmx? shadow? hap?).
>> 
>> If it's strictly toolstack/DM-driven maybe it'd be more apt to have a 
>> separate
>> LIVE_MIGRATION and SAVE_RESTORE configs where LM selects SAVE_RESTORE, which
>> selects LOG_DIRTY. That's also improve some defaults auto-downgraded from the
>> max policy just in case a VM is migrated.
>
> It's save (not restore) for both PV and HVM, and VRAM dirty tracking for HVM
> only. Ordinary HVM guests will want VRAM tracking, so compiling out support
> for it will imo want mentioning in the Kconfig help text.

ack.

>
>>>> --- a/xen/arch/x86/domctl.c
>>>> +++ b/xen/arch/x86/domctl.c
>>>> @@ -220,15 +220,15 @@ long arch_do_domctl(
>>>>      {
>>>>  
>>>>      case XEN_DOMCTL_shadow_op:
>>>> -#ifdef CONFIG_PAGING
>>>> +        ret = -EOPNOTSUPP;
>>>> +        if ( !IS_ENABLED(CONFIG_LOG_DIRTY) )
>>>> +            break;
>>>> +
>>>>          ret = paging_domctl(d, &domctl->u.shadow_op, u_domctl, 0);
>>>>          if ( ret == -ERESTART )
>>>>              return hypercall_create_continuation(
>>>>                         __HYPERVISOR_paging_domctl_cont, "h", u_domctl);
>>>>          copyback = true;
>>>> -#else
>>>> -        ret = -EOPNOTSUPP;
>>>> -#endif
>>>>          break;
>>>
>>> Can a HVM-only hypervisor create any guests with this? I simply fail to
>>> see how XEN_DOMCTL_SHADOW_OP_SET_ALLOCATION would then make it through to
>>> hap_domctl().
>> 
>> xl doesn't seem to call it at all. hap_set_allocation() is implicitly called
>> through paging_enable() -> hap_enable() -> hap_set_allocation()
>
> xl must be calling it, at least in the case where the paging pool size is
> explicitly set in the guest config. The important point is - not all of
> XEN_DOMCTL_shadow_op's sub-ops are log-dirty related.
>
> It's also odd that you did make changes at the call site here, but then
> left the called function (and its sibling paging_domctl_cont()) in place.
>
> Jan

That goes through DOMCTL_set_paging_mempool_size. With identical behaviour
except it uses a different op. Blergh.

There are no uses for shadow ops other than logdirty that I can see.

libs/guest/xg_sr_save.c:    rc = xc_shadow_control(xch, ctx->domid,
libs/guest/xg_sr_save.c-                           
XEN_DOMCTL_SHADOW_OP_ENABLE_LOGDIRTY,
--
libs/guest/xg_sr_save.c:        rc = xc_shadow_control(xch, ctx->domid, 
XEN_DOMCTL_SHADOW_OP_OFF,
libs/guest/xg_sr_save.c-                               NULL, 0);
--
libs/guest/xg_sr_save.c:            rc = xc_shadow_control(xch, ctx->domid,
libs/guest/xg_sr_save.c-                                   
XEN_DOMCTL_SHADOW_OP_ENABLE_LOGDIRTY,
--
libs/guest/xg_sr_save.c:    xc_shadow_control(xch, ctx->domid, 
XEN_DOMCTL_SHADOW_OP_OFF,
libs/guest/xg_sr_save.c-                      NULL, 0);
--
libs/light/libxl_colo_restore.c:    if (xc_shadow_control(CTX->xch, domid,
libs/light/libxl_colo_restore.c-                          
XEN_DOMCTL_SHADOW_OP_ENABLE_LOGDIRTY,
--
libs/light/libxl_colo_restore.c:    if (xc_shadow_control(CTX->xch, domid, 
XEN_DOMCTL_SHADOW_OP_OFF,
--
ocaml/libs/xc/xenctrl_stubs.c:  ret = xc_shadow_control(xch, Int_val(domid),
ocaml/libs/xc/xenctrl_stubs.c-                          
XEN_DOMCTL_SHADOW_OP_GET_ALLOCATION,
--
ocaml/libs/xc/xenctrl_stubs.c:  ret = xc_shadow_control(xch, Int_val(domid),
ocaml/libs/xc/xenctrl_stubs.c-                          
XEN_DOMCTL_SHADOW_OP_SET_ALLOCATION,

Not even for shadow, which seems 

There's an ocaml stub, but I can't be bothered to check whether XAPI uses it.

I'll leave around the domctl so XEN_DOMCTL_SHADOW_OP_OFF doesn't return
-EOPNOTSUPP, but set/get allocation should go away at some point before
stabilising the control interfaces.

I'll also remove the bad #if PG_log_dirty gating these, as it really just wants
to be !PV_SHIM_EXCLUSIVE (that's why I originally got rid of it all).

Cheers,
Alejandro