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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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;


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.

~Andrew