Re: [Xen-devel] [PATCH v7 1/3] x86/tlb: introduce a flush HVM ASIDs flag

["Roger Pau Monné" <roger.pau@citrix.com>]

On Fri, Mar 20, 2020 at 02:43:38PM +0000, Julien Grall wrote:
> Hi,
> 
> On 20/03/2020 14:27, Julien Grall wrote:
> > 
> > 
> > On 20/03/2020 14:22, Roger Pau Monné wrote:
> > > static inline void filtered_flush_tlb_mask(uint32_t tlbflush_timestamp)
> > > {
> > >      cpumask_t mask;
> > > 
> > >      cpumask_copy(&mask, &cpu_online_map);
> > >      tlbflush_filter(&mask, tlbflush_timestamp);
> > >      if ( !cpumask_empty(&mask) )
> > >      {
> > >          perfc_incr(need_flush_tlb_flush);
> > > #if CONFIG_X86
> > >          /*
> > >           * filtered_flush_tlb_mask is used after modifying the p2m in
> > >           * populate_physmap, Xen needs to trigger an ASID tickle as
> > > this is a
> > >           * requirement on AMD hardware.
> > >           */
> > 
> > I don't think this comment is correct. populate_physmap() is only going
> > to add entry in the P2M and therefore flush should not be needed.
> 
> I should have probably said "in most of the cases..." and ...
> 
> > 
> > The only reason the flush would happen in populate_physmap() is because
> > we allocated a page that was required to be flush (see
> > free.need_tbflush).
> 
> ... extend this comment a bit more. The flush will happen when the page used
> to have an owner. So if there is no owner, there is no flush.
> 
> Therefore we can't rely on it if we really wanted to trigger an ASID tickle
> after a P2M update.

Right, so I can leave filtered_flush_tlb_mask as-is. Will prepare a
new patch.

Thanks, Roger.

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