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Re: [Xen-devel] [PATCH v3 4/7] x86/nospec: Rename and rework l1tf-barrier as branch-harden
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 29 Oct 2019 17:00:12 +0000
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Tue, 29 Oct 2019 17:00:24 +0000
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On 25/10/2019 13:09, Jan Beulich wrote:
> On 23.10.2019 15:58, Andrew Cooper wrote:
>> l1tf-barrier is an inappropriate name, and came about because of restrictions
>> on could be discussed publicly when the patches were proposed.
>>
>> In practice, it is for general Spectre v1 mitigations, and is necessary in
>> all
>> cases. An adversary which can control speculation in Xen can leak data in
>> cross-core (BCBS, etc) or remote (NetSpectre) scenarios - the problem is not
>> limited to just L1TF with HT active.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>
>> In principle it should be tristate and being disabled by default on parts
>> which don't speculate, but it is too late in 4.13 to organise this.
> And the fundamental issue is correctly compiling the conditions under which
> to default to true and false respectively?
Yes.
> I ask because if it was not this
> then I wouldn't see what hindering factor there is to make this tristate
> right away.
Linux's NO_SPECULATION list is a start (if there is actually an
intersection with 64bit capable CPUs), but there are mitigating factors
on some later CPUs as well.
~Andrew
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