Re: [PATCH v3 4/8] xen: Use typesafe gfn in xenmem_add_to_physmap_one

From: Julien Grall <julien.grall@arm.com>
To: Stefano Stabellini <sstabellini@kernel.org>
Cc: Tim Deegan <tim@xen.org>, Wei Liu <wei.liu2@citrix.com>,
	George Dunlap <george.dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Steve Capper <Steve.Capper@arm.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	xen-devel@lists.xen.org, Jan Beulich <jbeulich@suse.com>
Subject: Re: [PATCH v3 4/8] xen: Use typesafe gfn in xenmem_add_to_physmap_one
Date: Thu, 23 Jun 2016 14:33:55 +0100	[thread overview]
Message-ID: <576BE543.7050801@arm.com> (raw)
In-Reply-To: <alpine.DEB.2.10.1606231403420.2575@sstabellini-ThinkPad-X260>

On 23/06/16 14:06, Stefano Stabellini wrote:
> On Thu, 23 Jun 2016, Julien Grall wrote:
>> On 23/06/16 11:20, Stefano Stabellini wrote:
>>> On Tue, 21 Jun 2016, Julien Grall wrote:
>>> I think there is a possible loss of information here: xen_pfn_t is
>>> always uint64_t on both ARM and ARM64, while gfn_t is unsigned long with
>>> its current definition. Or am I missing something?
>>>
>>> In fact, given that ARM supports LPAE, shouldn't gfn by defined as
>>> xen_pfn_t in terms of storage size (TYPE_SAFE(xen_pfn_t, gfn)) ?
>>
>> With LPAE, ARM32 supports up to 40-bit PA so the frame will be encoded on
>> 28-bit. So unsigned long is perfectly fine here.
>
> Somehow I have always assumed that the 40-bit limitation was just
> temporary. That ARM in the future will just increase that number up to
> 64-bit eventually.
>
> If you don't think there is any risk of that happening, then I am fine
> with this. We just have to keep in mind that many of the guest
> interfaces use xen_pfn_t, which has a different size on ARM.

Currently, Aarch32 supports up to 40-bit whilst Aarch64 supports up to 
48-bit (even 52-bit with ARMv8.2). So this should be ok for now.

However, pretty much everywhere in Xen we assume that the frame number 
is unsigned long (see mm.c, p2m.c,...). We would have much more work to 
do than this small patch.

I would rather start to switch to gfn/mfn internally and keep the 
underlying type as "unsigned long" until we effectively need 64-bit frame.

The main reason is 64-bit frame will result into a bigger binary for 
ARM32 with no apparent reason (40-bit is hardcoded in 
setup_virt_paging). Switching to gfn/mfn will allow us to uint64_t where 
it will be required.

Regards,

-- 
Julien Grall

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