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

From: Stefano Stabellini <sstabellini@kernel.org>
To: Julien Grall <julien@xen.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	 xen-devel@lists.xenproject.org, Wei.Chen@arm.com,
	Henry.Wang@arm.com,  Penny.Zheng@arm.com,
	Bertrand.Marquis@arm.com,  Julien Grall <jgrall@amazon.com>,
	 Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Subject: Re: [PATCH RFCv2 10/15] xen/arm: mm: Allocate xen page tables in domheap rather than xenheap
Date: Thu, 13 May 2021 15:27:42 -0700 (PDT)	[thread overview]
Message-ID: <alpine.DEB.2.21.2105131522030.5018@sstabellini-ThinkPad-T480s> (raw)
In-Reply-To: <9429bec0-8706-42b9-cda6-77adde961235@xen.org>

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?

The only difference is that on arm32 we are using domheap to allocate
the pages, which is a different (larger) pool.