Re: [Xen-devel] RT Xen on ARM - R-Car series

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

On 05.02.19 00:06, Julien Grall wrote:
> > A57+A53.
> >
> > I see the following in my log:
> >
> >      (XEN) alternatives: Patching with alt table 00000000002c6608 -> 
> > 00000000002c6c80
> >      (XEN) CPU0 will call ARM_SMCCC_ARCH_WORKAROUND_1 on exception entry
> >      (XEN) CPU2 will call ARM_SMCCC_ARCH_WORKAROUND_1 on exception entry
> >      (XEN) CPU3 will call ARM_SMCCC_ARCH_WORKAROUND_1 on exception entry
> >      (XEN) CPU1 will call ARM_SMCCC_ARCH_WORKAROUND_1 on exception entry
> Cortex-A53 should not be affected by spectre v2, so I imagine they are only for 
> A57?
Yes, the log says the workaround is applied to the big cores only. As it should 
be.

> It is going to be hard to disable the workarounds by default. But we can 
> consider to provide host-wide or per-guest option to disable them on trusted 
> environment.
We have to get numbers first than decide how to proceed.

> > > Also, when you mean possible, does it mean you haven't looked the performance 
> > > regression?
> > We have a preliminary results about performance drop with xen4.12-unstable 
> > comparing to a our system with 4.10.
> A lot of patches have not been backported in Xen 4.10 (including 
> Spectre/Meltdown) that will definitely fix hole but may have an impact on the 
> performance. There were not backported because of performance reason but 
> because of the complexity of the port and seemly lack of interest.
I know that story. But customers are customers. And performance drop in the 
next SW version is always painful for them.
So we need a good explanation (which Spectre mitigation might be), or better to 
show up no performance drop :).

--
Sincerely,
Andrii Anisov.

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