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

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

On Fri, Mar 20, 2026 at 09:44:39AM +0000, Ross Lagerwall wrote:
> On 3/19/26 2:06 PM, Roger Pau Monné wrote:
> > 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.
> > 
> 
> Yes, that makes sense. Can the adjustment be done on commit if there is
> no other feedback?

Sure, I can take care of that at commit time if there are no further
comments.

Thanks, Roger.