Re: [Xen-devel] [PATCH 09/14] xen: Introduce HAS_M2P config and use to protect mfn_to_gmfn call

From: "Jan Beulich" <JBeulich@suse.com>
To: "Julien Grall" <julien.grall@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <Ian.Jackson@eu.citrix.com>, Tim Deegan <tim@xen.org>,
	oleksandr_tyshchenko@epam.com,
	xen-devel <xen-devel@lists.xenproject.org>,
	"andrii_anisov@epam.com" <andrii_anisov@epam.com>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH 09/14] xen: Introduce HAS_M2P config and use to protect mfn_to_gmfn call
Date: Fri, 10 May 2019 08:19:31 -0600	[thread overview]
Message-ID: <5CD58873020000780022D9FC@prv1-mh.provo.novell.com> (raw)
Message-ID: <20190510141931.ysACByrzNpdr7EpYjNQyL_mTIbW-TcoZEVx56IgCyk8@z> (raw)
In-Reply-To: <9e696d26-0d3b-da9e-7427-f29da07dde7c@arm.com>

>>> On 10.05.19 at 16:04, <julien.grall@arm.com> wrote:
> On 10/05/2019 14:45, Jan Beulich wrote:
>>>>> On 10.05.19 at 15:41, <julien.grall@arm.com> wrote:
>>> The point here, we keep within the hypervisor the idea of what's valid or
>>> invalid. This allows us more flexibility on the value here (imagine we decide to
>>> change the value of GFN_INVALID in the future...).
>> 
>> Exactly, and hence INVALID_GFN should not become visible to the
>> outside. Hence my request to use an all-ones value here.
> It is only visible if you put an exact value in the documentation. Your 
> suggestion is to put a exactly value and I would rather not see that.

I did specifically suggest to _not_ store INVALID_GFN here, but to
store 64-bit bits of ones. Note the difference between the two on
32-bit Arm.

>>>>> Well, it at least tell you the function can't work. So I think it is still makes
>>>>> sense to have it.
>>>>
>>>> I disagree.
>>> You disagree because...?
>> 
>> Because of what I've said in my initial reply (still quoted above).
> 
> I still don't see the problem of unconditional log message. It is not really the 
> first place we have that.
> 
>> 
>>> I hope you are aware, this is unlikely going to be printed as the code should
>>> not be called.
>> 
>> ASSERT_UNREACHABLE() then?
> 
> And still avoiding the printk?

Preferably yes; depends on how exactly you code the assertion.
If you follow the if()-ASSERT_UNREACHABLE()-return style we've
been using elsewhere, then no matter how you place the #else
or #endif the printk() will be compiled out.

Jan

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