Re: [PATCH v2] x86/hap: Defer NPT P2M TLB flushes

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

On Thu, Mar 19, 2026 at 11:40:41AM +0000, Ross Lagerwall wrote:
> Like the EPT code, defer TLB flushes for NPT 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>
> ---
> In v2:
> * Tweak commit message.
> * Call guest_flush_tlb_mask() if the assertion fails.
> 
>  xen/arch/x86/mm/hap/hap.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
> index a337752bf488..67137611d9db 100644
> --- a/xen/arch/x86/mm/hap/hap.c
> +++ b/xen/arch/x86/mm/hap/hap.c
> @@ -814,15 +814,33 @@ 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)
>  {
> -    struct domain *d = p2m->domain;
> +    if ( !(oflags & _PAGE_PRESENT) )
> +        return;
> +
> +    if ( unlikely(!p2m->defer_flush) )
> +    {
> +        struct domain *d = p2m->domain;

As you are moving this around, and seeing that guest_flush_tlb_mask()
takes a const domain parameter, I think you could make this local
variable const.  Possibly the same below with the other d local
variable.

With that:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.