Re: [PATCH] x86/hap: Defer P2M TLB flushes

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

The subject should possibly mention NPT, as reading HAP one would
assume it applies to both EPT and NPT (and the EPT side is already
done here):

x86/hap: Defer NPT P2M TLB flushes

On Wed, Mar 18, 2026 at 05:13:24PM +0000, Ross Lagerwall wrote:
> Like the EPT code, defer TLB flushes to reduce the number of flushes and
> avoid holding the P2M lock while flushing. This can substantially
> improve performance in some scenarios.
> 
> The cases where the TLB needs to be flushed without deferring are
> already handled by the call to p2m_tlb_flush_sync() in p2m_free_ptp().
> 
> Suggested-by: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> ---
>  xen/arch/x86/mm/hap/hap.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
> index a337752bf488..1eba995c7b39 100644
> --- a/xen/arch/x86/mm/hap/hap.c
> +++ b/xen/arch/x86/mm/hap/hap.c
> @@ -813,16 +813,26 @@ static void cf_check hap_update_paging_modes(struct 
> vcpu *v)
>  
>  static void cf_check
>  hap_write_p2m_entry_post(struct p2m_domain *p2m, unsigned int oflags)
> +{
> +    if ( oflags & _PAGE_PRESENT )
> +    {
> +        ASSERT(p2m->defer_flush);

Here I'm unsure whether we might want to do:

if ( !p2m->defer_flush )
{
    ASSERT_UNREACHABLE();
    guest_flush_tlb_mask(d, d->dirty_cpumask);
}

Instead of just asserting that the defer_flush field is set, to be on
the safe(r) side.

Thanks, Roger.