Re: [PATCH 2/2] x86: use pv-ops in {pte,pmd}_{set,clear}_flags()

[Pavel Emelyanov <xemul@parallels.com>]

On 04/01/2014 11:03 PM, Cyrill Gorcunov wrote:
> On Tue, Apr 01, 2014 at 11:43:11AM -0700, Linus Torvalds wrote:
>> On Tue, Apr 1, 2014 at 11:18 AM, David Vrabel <david.vrabel@citrix.com> wrote:
>>>
>>> I don't think it's sufficient to avoid collisions with bits used only
>>> with P=0.  The original value of this bit must be retained when the
>>> _PAGE_NUMA bit is set/cleared.
>>>
>>> Bit 7 is PAT[2] and whilst Linux currently sets up the PAT such that
>>> PAT[2] is a 'don't care', there has been talk up adjusting the PAT to
>>> include more types. So I'm not sure it's a good idea to use bit 7.
>>>
>>> What's wrong with using e.g., bit 62? And not supporting this NUMA
>>> rebalancing feature on 32-bit non-PAE builds?
>>
>> Sounds good to me, but it's not available in 32-bit PAE. The high bits
>> are all reserved, afaik.
>>
>> But you'd have to be insane to care about NUMA balancing on 32-bit,
>> even with PAE. So restricting it to x86-64 and using the high bits (I
>> think bits 52-62 are all available to SW) sounds fine to me.
>>
>> Same goes for soft-dirty. I think it's fine if we say that you won't
>> have soft-dirty with a 32-bit kernel. Even with PAE.
> 
> Well, at the moment we use soft-dirty for x86-64 only in criu but there
> were plans to implement complete 32bit support as well. While personally
> I don't mind dropping soft-dirty for non x86-64 case, I would like
> to hear Pavel's opinion, Pavel?

We (Parallels) don't have plans on C/R on 32-bit kernels, but I speak only
for Parallels. However, people I know who need 32-bit C/R use ARM :)

> (n.b, i'm still working on cleaning up _page bits, it appeared to
>  be harder than I've been expecting).
> .

Thanks,
Pavel

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