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Re: [Xen-devel] [PATCH] x86: undo part of "refine link time stub area related assertion"
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 30 Jan 2020 11:43:52 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Thu, 30 Jan 2020 11:44:06 +0000
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On 30/01/2020 08:09, Jan Beulich wrote:
> On 29.01.2020 18:14, Andrew Cooper wrote:
>> On 29/01/2020 17:03, Jan Beulich wrote:
>>> The original check was not too strict: While we don't use one page of
>>> memory per CPU, we do use ons page of VA space per CPU. It is the
>> one.
>>
>>> latter which matters here.
>>>
>>> Undo that part of the change, but leave everything else in place.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> Ok, but this begs the question why? If the stubs are tightly packed
>> together, but the linear space isn't, we end up having loads of aliases
>> of the stubs.
>>
>> There is no security benefit for doing so, but there is a real
>> performance cost from not compacting the linear space. Most notably,
>> two threads unable to share a 4k tlb entry for their own stubs, but also
>> reduced cache locality of reference for the pagetables requires to map
>> the overly-large linear space.
> The idea, iirc, was to make the addresses even more easily
> recognizable this way, in particular in the case a stub was
> referenced for a CPU that was taken offline. But yes, this isn't
> an overly big win in this regard, so perhaps the arrangement
> could be revised (looking over the code there don't look to be
> any other dependencies on this layout). Until then though the
> assertion should be fixed, as right now it is clearly
> insufficient.
For the patch, Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Improvements can be done at a later point.
~Andrew
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