Re: [Xen-devel] [PATCH v3 2/2] x86/setup: remap Xen image up to PFN_DOWN(__pa(_end))

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.02.18 at 14:46, <daniel.kiper@xxxxxxxxxx> wrote:
> Sorry for late reply but I was busy with other stuff.
> 
> On Fri, Jan 19, 2018 at 08:27:46AM -0700, Jan Beulich wrote:
>> >>> On 10.01.18 at 14:05, <daniel.kiper@xxxxxxxxxx> wrote:
>> > Current limit, PFN_DOWN(xen_phys_start), introduced by commit b280442
>> > (x86: make Xen early boot code relocatable) is not reliable. Potentially
>> > its value may fall below PFN_DOWN(__pa(_end)) and then part of Xen image
>> > may not be mapped after relocation. This will not happen in current code
>> > thanks to "x86/setup: do not relocate over current Xen image placement"
>> > patch. Though this safety measure may save a lot of debugging time when
>> > somebody decide to relax existing relocation restrictions one day.
>>
>> I've gone back through the v2 discussion, and I continue to fail to
>> see what is being fixed here, even if just theoretically. It is bad
> 
> OK, let's give an example. I assume that there is no patch 1 and Xen can
> relocate itself even it was initially relocated by the bootloader. So,
> let's assume that the bootloader loaded Xen image at 0x80200000
> (xen_phys_start == 0x80000000) and its size is 0x700000 (7 MiB).
> The RAM region ends at 0x80D00000 and there is no RAM above that
> address. At some point Xen realizes that it can relocate itself
> to 0x80600000 (xen_phys_start == 0x80400000). So, it does and then
> remaps itself. And here is the problem. Currently existing code
> will remap only Xen image up to 0x803fffff. Everything above will
> no be remapped. So, that is why I suggested this patch.
> 
>> enough that the description here isn't clarifying this and I need to
>> go back to the earlier discussion, but it's even worse if even that
>> earlier discussion didn't really help. My conclusion is that you're
> 
> Sorry about that.
> 
>> talking about a case where old and positions of Xen overlap, a
>> case which I thought patch 1 eliminates.
> 
> It does not eliminate the issue described above. It just hides it.

Well, no, I disagree - it makes an overlap impossible afaict,
which is more that just hiding the problem. Anyway - I'm not
going to object to the change provided it comes with a clear
description of what _existing_ issue (even if just a theoretical
one) is being fixed _with the currently present code in mind_
(i.e. in particular including your patch 1).

Jan


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