Re: [PATCH v3] x86/vmx: Avoid pausing on HVM_PARAM_IDENT_PT in additional cases

[Teddy Astie <teddy.astie@xxxxxxxxxx>]

Le 09/09/2026 à 17:35, Andrew Cooper a écrit :
> On 09/09/2026 1:12 pm, Teddy Astie wrote:
> > When settings HVM_PARAM_IDENT_PT, skip domain pausing when :
> > - there is no vcpu
> > - unrestricted guest capability is used
> >
> > Signed-off-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
> > ---
> > Regarding checking for hvm_paging_enabled(v) (proposed in v2 review), we can't
> > as hvm_paging_enabled() is vCPU specific, and vCPU 0 may have a different value
> > to another one.
> >
> > v3:
> >   - rebased patches with staging
> >   - adjusted formatting
> >
> > v2:
> >   - rebased patches with staging
> >
> >   xen/arch/x86/hvm/hvm.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> > index 9a4147b62e..2981a61cee 100644
> > --- a/xen/arch/x86/hvm/hvm.c
> > +++ b/xen/arch/x86/hvm/hvm.c
> > @@ -4237,11 +4237,14 @@ static int hvm_set_param(struct domain *d, uint32_t 
> > index, uint64_t value)
> >               rc = -EINVAL;
> >           break;
> >       case HVM_PARAM_IDENT_PT:
> > +        v = domain_vcpu(d, 0);
> > +
> >           /*
> >            * Only actually required for VT-x lacking unrestricted_guest
> >            * capabilities.  Short circuit the pause if possible.
> >            */
> > -        if ( paging_mode_shadow(d) || !using_vmx() )
> > +        if ( paging_mode_shadow(d) || !using_vmx() || !v ||
> > +             vmx_unrestricted_guest(v) )
> >               break;
> The comment isn't really correct, and using vmx_unrestricted_guest()
> isn't really correct either.
>
> How about this instead:
>
>          /*
>           * IDENT_PT is needed only for Nehalem-era VT-x, where EPT is
>           * available but Unrestricted Guest is not.
>           *
>           * The identity pagetable is configured by the toolstack or hvmloader,
>           * and the param needs to move in the migration stream in case the VM
>           * lands on an EPT && !Unrestricted system.
>           *
>           * Nothing, besides recording the value, needs to happen other than
>           * for VT-x EPT && !Unrestricted VMs.
>           *
>           * TODO: Unrestricted Guest should be a domain property not a vCPU
>           * property.
>           */
>          v = domain_vcpu(d, 0);
>          if ( !using_vmx() || !paging_mode_hap(d) || !v ||
>               vmx_unrestricted_guest(v) )
>              break;
I'm ok with it (though regarding paging_mode_hap, I think 
paging_mode_shadow is prefered here because it will translate into 
directly 0 if shadow paging is disabled, otherwise we check for 
PG_HAP_enable, at least, that's what [1] seems to be about).
But if we move unrestricted guest as being a domain-wide properly (or 
alternatively a system-wide one); that could make things simpler.
[1] x86/paging: replace !paging_mode_hap() with paging_mode_shadow()

> The case of no vCPUs isn't very interesting; in that case, pausing the
> domain is free.  We can at least note that the data is in the wrong
> place, even if we don't fix it yet.
The check for !v is mostly required for vmx_unrestricted_guest(v) to not 
dereference NULL.
> ~Andrew
Teddy

Attachment: OpenPGP_signature.asc
Description: OpenPGP digital signature