Re: Error during update_runstate_area with KPTI activated

[Julien Grall <julien@xxxxxxx>]

On 14/05/2020 15:28, Bertrand Marquis wrote:
> Hi,
Hi,

> When executing linux on arm64 with KPTI activated (in Dom0 or in a DomU), I 
> have a lot of walk page table errors like this:
> (XEN) p2m.c:1890: d1v0: Failed to walk page-table va 0xffffff837ebe0cd0
>
> After implementing a call trace, I found that the problem was coming from the 
> update_runstate_area when linux has KPTI activated.
>
> I have the following call trace:
> (XEN) p2m.c:1890: d1v0: Failed to walk page-table va 0xffffff837ebe0cd0
> (XEN) backtrace.c:29: Stacktrace start at 0x8007638efbb0 depth 10
> (XEN)    [<000000000027780c>] get_page_from_gva+0x180/0x35c
> (XEN)    [<00000000002700c8>] guestcopy.c#copy_guest+0x1b0/0x2e4
> (XEN)    [<0000000000270228>] raw_copy_to_guest+0x2c/0x34
> (XEN)    [<0000000000268dd0>] domain.c#update_runstate_area+0x90/0xc8
> (XEN)    [<000000000026909c>] domain.c#schedule_tail+0x294/0x2d8
> (XEN)    [<0000000000269524>] context_switch+0x58/0x70
> (XEN)    [<00000000002479c4>] core.c#sched_context_switch+0x88/0x1e4
> (XEN)    [<000000000024845c>] core.c#schedule+0x224/0x2ec
> (XEN)    [<0000000000224018>] softirq.c#__do_softirq+0xe4/0x128
> (XEN)    [<00000000002240d4>] do_softirq+0x14/0x1c
>
> Discussing this subject with Stefano, he pointed me to a discussion started a 
> year ago on this subject here:
> https://lists.xenproject.org/archives/html/xen-devel/2018-11/msg03053.html
>
> And a patch was submitted:
> https://lists.xenproject.org/archives/html/xen-devel/2019-05/msg02320.html
>
> I rebased this patch on current master and it is solving the problem I have 
> seen.
>
> It sounds to me like a good solution to introduce a 
> VCPUOP_register_runstate_phys_memory_area to not depend on the area actually 
> being mapped in the guest when a context switch is being done (which is 
> actually the problem happening when a context switch is trigger while a guest 
> is running in EL0).
>
> Is there any reason why this was not merged at the end ?
I just skimmed through the thread to remind myself the state. AFAICT, 
this is blocked on the contributor to clarify the intended interaction 
and provide a new version.
I am still in favor of the new hypercall (and still in my todo list) but 
I haven't yet found time to revive the series.
Would you be willing to take over the series? I would be happy to bring 
you up to speed and provide review.
Cheers,

--
Julien Grall