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Re: [Xen-devel] [PATCH 39/60] x86: optimize loading of GDT at context switch
- To: Juergen Gross <jgross@xxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 3 Jul 2019 13:21:42 +0100
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- Cc: Wei Liu <wl@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 03 Jul 2019 12:21:52 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 03/07/2019 07:30, Juergen Gross wrote:
> On 02.07.19 18:09, Andrew Cooper wrote:
>> On 28/05/2019 11:32, Juergen Gross wrote:
>>> Instead of dynamically decide whether the previous vcpu was using full
>>
>> "deciding"
>>
>>> or default GDT just add a percpu variable for that purpose. This at
>>
>> "was using a full or default GDT, just add"
>>
>>> once removes the need for testing vcpu_ids to differ twice.
>>>
>>> Cache the need_full_gdt(nd) value in a local variable.
>>
>> What's the point of doing this? I know the logic is rather complicated
>> in __context_switch(), but at least it is visually consistent. After
>> this change, it is asymmetric and harder to follow.
>
> This is a hot path. need_full_gdt() needs two compares, of which one is
> using evaluate_nospec().
Urgh. So evalute_nospec() is already broken here because
need_full_gdt() isn't always_inline, but surely this isn't the only
example impacted in __context_switch()? The choice of 'gdt' is
similarly impacted by the looks of things.
I'd recommend not worrying about evalute_nospec() for now. There are
several fundamental problems atm, and Xen 4.13 cannot ship with it in
this state.
~Andrew
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