Re: [Xen-devel] [PATCH for-4.12] arm/p2m: do not invalidate p2m root if it is shared with IOMMU

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

Hello Julien,

Actually I was going to send this patch as RFC, but dropped it at the last 
moment.

On 28.01.19 17:55, Julien Grall wrote:
> This was missed on purpose. Let me explain why. The call to 
> p2m_invalidate_root() arch_domain_creation_finished is called by *all* the 
> domain at boot to try to optimize the set/way case.
>
> The check iommu_use_hap_pt in that context is to prevent guest not using 
> Set/Way to become unusable under the IOMMU use-case.
>
> In your case, you seem to have a guest OS using set/way and yet sharing the P2M 
> with the IOMMU. You have the choice between:
>      1) Crashing on IOMMU fault
>      2) Become very slow and potentially unusable because you now have to go 
> through the full P2M every time you do a Set/Way.
>
> 1) was my favored option because Set/Way should really not be used by the 
> guest. It was implemented by courtesy to the guest OS and I would not rely on 
> everything working.
>
> Can you explain what is your use-case (OS used, IOMMU, platform...)
Yep, I'm nearly finished moving our development setup to XEN 4.12-rc.
The platform is R-Car Gen3 (CA57+CA53), Renesas'es IOMMU named IPMMU is 
utilized for all peripherals assigned to DomD and GPU shared between DomD and 
DomA.
Three OS'es are running on the HW: HW-less Dom0, driver domain DomD with GPU 
sharing, Android running on PV drivers and GPU sharing.
So on system start (DomD booting) I see few translation faults from DomD only 
assigned peripherals. But then Android starts and there are lot of translation 
faults from GPU trying access DomA memory. Then GPU FW dies.

--
Sincerely,
Andrii Anisov.

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