linux-kernel - Re: [RFC PATCH v2 16/26] KVM: arm64: Prepare Hyp memory protection

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Message-ID: <20210205175602.GG22665@willie-the-truck>
Date:   Fri, 5 Feb 2021 17:56:02 +0000
From:   Will Deacon <will@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, android-kvm@...gle.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        Fuad Tabba <tabba@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        David Brazdil <dbrazdil@...gle.com>
Subject: Re: [RFC PATCH v2 16/26] KVM: arm64: Prepare Hyp memory protection

On Thu, Feb 04, 2021 at 10:47:08AM +0000, Quentin Perret wrote:
> On Wednesday 03 Feb 2021 at 14:37:10 (+0000), Will Deacon wrote:
> > On Fri, Jan 08, 2021 at 12:15:14PM +0000, Quentin Perret wrote:
> > > +static inline unsigned long __hyp_pgtable_max_pages(unsigned long nr_pages)
> > > +{
> > > +	unsigned long total = 0, i;
> > > +
> > > +	/* Provision the worst case scenario with 4 levels of page-table */
> > > +	for (i = 0; i < 4; i++) {
> > 
> > Looks like you want KVM_PGTABLE_MAX_LEVELS, so maybe move that into a
> > header?
> 
> Will do.
> 
> > 
> > > +		nr_pages = DIV_ROUND_UP(nr_pages, PTRS_PER_PTE);
> > > +		total += nr_pages;
> > > +	}
> > 
> > ... that said, I'm not sure this needs to iterate at all. What exactly are
> > you trying to compute?
> 
> I'm trying to figure out how many pages I will need to construct a
> page-table covering nr_pages contiguous pages. The first iteration tells
> me how many level 0 pages I need to cover nr_pages, the second iteration
> how many level 1 pages I need to cover the level 0 pages, and so on...

Ah, you iterate from leaves back to the root. Got it, thanks.

> I might be doing this naively though. Got a better idea?

I thought I did, but I ended up with something based on a geometric series
and it looks terrible to code-up in C without, err, iterating like you do.

So yeah, ignore me :)

> > > +
> > > +	return total;
> > > +}
> > > +
> > > +static inline unsigned long hyp_s1_pgtable_size(void)
> > > +{
> > > +	struct hyp_memblock_region *reg;
> > > +	unsigned long nr_pages, res = 0;
> > > +	int i;
> > > +
> > > +	if (kvm_nvhe_sym(hyp_memblock_nr) <= 0)
> > > +		return 0;
> > 
> > It's a bit grotty having this be signed. Why do we need to encode the error
> > case differently from the 0 case?
> 
> Here specifically we don't, but it is needed in early_init_dt_add_memory_hyp()
> to distinguish the overflow case from the first memblock being added.

Fair enough, but if you figure out a way for hyp_memblock_nr to be unsigned,
I think that would be preferable.

Will