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

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

On 04.02.19 13:36, Julien Grall wrote:
> That's a good news! Let me try to address your concerns below one by one.
Lets do it:)

> > And they employ KPTI enabled kernel in the BSP.
> KPTI is going to work on Xen. There are no known issue with Linux as the 
> virtual address is not going to be re-used for other purpose in the virtual 
> address space. The only inconvenience is the message in debug build.
> Just in case, I am not saying it should not be fixed :).
We have a decision to eliminate it.

> > That reveals another critical issue for us, in addition to Set/Way issue 
>  From the discussion on the another thread and with other people, this is not 
> entirely the fault of Xen. This was a misuse of the instructions by the driver.
Yep, but it is still on our team to solve.

> While you may want to deal with this in your case, I would like to avoid 
> promoting bad behavior when using Xen upstream.
Ah, ok. Will say that in different words.

> > and possible performance drops/irq latency raise due to specter mitigation 
> > measures.
> Can you remind me the cores you are using?
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

> 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.
But those results are from Nov IIRC, and we have to recheck. After fixing 
Set/Way misuse in our proprietary driver ;)

> > I'm not sure if this is sufficient a justification to make it the release 
> > blocker, but we are up to this stuff.
> Which one?
KPTI issue, as you suggested.

--
Sincerely,
Andrii Anisov.

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