Re: [PATCH RFCv2 10/15] xen/arm: mm: Allocate xen page tables in domheap rather than xenheap

[Stefano Stabellini <sstabellini@kernel.org>]

On Sat, 15 May 2021, Julien Grall wrote:
> Hi Stefano,
> 
> On 13/05/2021 23:27, Stefano Stabellini wrote:
> > On Thu, 13 May 2021, Julien Grall wrote:
> > > Hi Stefano,
> > > 
> > > On 12/05/2021 23:44, Stefano Stabellini wrote:
> > > > On Sun, 25 Apr 2021, Julien Grall wrote:
> > > > > From: Julien Grall <jgrall@amazon.com>
> > > > > 
> > > > > xen_{un,}map_table() already uses the helper to map/unmap pages
> > > > > on-demand (note this is currently a NOP on arm64). So switching to
> > > > > domheap don't have any disavantage.
> > > > > 
> > > > > But this as the benefit:
> > > > >       - to keep the page tables unmapped if an arch decided to do so
> > > > >       - reduce xenheap use on arm32 which can be pretty small
> > > > > 
> > > > > Signed-off-by: Julien Grall <jgrall@amazon.com>
> > > > 
> > > > Thanks for the patch. It looks OK but let me ask a couple of questions
> > > > to clarify my doubts.
> > > > 
> > > > This change should have no impact to arm64, right?
> > > > 
> > > > For arm32, I wonder why we were using map_domain_page before in
> > > > xen_map_table: it wasn't necessary, was it? In fact, one could even say
> > > > that it was wrong?
> > > In xen_map_table() we need to be able to map pages from Xen binary,
> > > xenheap...
> > > On arm64, we would be able to use mfn_to_virt() because everything is
> > > mapped
> > > in Xen. But that's not the case on arm32. So we need a way to map anything
> > > easily.
> > > 
> > > The only difference between xenheap and domheap are the former is always
> > > mapped while the latter may not be. So one can also view a xenheap page as
> > > a
> > > glorified domheap.
> > > 
> > > I also don't really want to create yet another interface to map pages (we
> > > have
> > > vmap(), map_domain_page(), map_domain_global_page()...). So, when I
> > > implemented xen_map_table() a couple of years ago, I came to the
> > > conclusion
> > > that this is a convenient (ab)use of the interface.
> > 
> > Got it. Repeating to check if I see the full picture. On ARM64 there are
> > no changes. On ARM32, at runtime there are no changes mapping/unmapping
> > pages because xen_map_table is already mapping all pages using domheap,
> > even xenheap pages are mapped as domheap; so this patch makes no
> > difference in mapping/unmapping, correct?
> 
> For arm32, it makes a slight difference when allocating a new page table (we
> didn't call map/unmap before) but this is not called often.
> 
> The main "drop" in performance happened when xen_{,map}_table() was
> introduced.
> 
> > 
> > The only difference is that on arm32 we are using domheap to allocate
> > the pages, which is a different (larger) pool.
> 
> Yes.
> 
> Would you be happy to give you acked-by/reviewed-by on this basis?

Yes

Acked-by: Stefano Stabellini <sstabellini@kernel.org>