Re: [RFC PATCH 00/10] Preemption in hypervisor (ARM only)

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

Hi,

Volodymyr Babchuk writes:

> Hi Andrew,
>
> Andrew Cooper writes:
>
>> On 24/02/2021 23:58, Volodymyr Babchuk wrote:
>>> And I am not mentioning x86 support there...
>>
>> x86 uses per-pCPU stacks, not per-vCPU stacks.
>>
>> Transcribing from an old thread which happened in private as part of an
>> XSA discussion, concerning the implications of trying to change this.
>>
>> ~Andrew
>>
>> -----8<-----
>>
>> Here is a partial list off the top of my head of the practical problems
>> you're going to have to solve.
>>
>> Introduction of new SpectreRSB vulnerable gadgets.  I'm really close to
>> being able to drop RSB stuffing and recover some performance in Xen.
>>
>> CPL0 entrypoints need updating across schedule.  SYSCALL entry would
>> need to become a stub per vcpu, rather than the current stub per pcpu.
>> This requires reintroducing a writeable mapping to the TSS (doable) and
>> a shadow stack switch of active stacks (This corner case is so broken it
>> looks to be a blocker for CET-SS support in Linux, and is resulting in
>> some conversation about tweaking Shstk's in future processors).
>>
>> All per-cpu variables stop working.  You'd need to rewrite Xen to use
>> %gs for TLS which will have churn in the PV logic, and introduce the x86
>> architectural corner cases of running with an invalid %gs.  Xen has been
>> saved from a large number of privilege escalation vulnerabilities in
>> common with Linux and Windows by the fact that we don't use %gs, so
>> anyone trying to do this is going to have to come up with some concrete
>> way of proving that the corner cases are covered.
>
> Thank you. This is exactly what I needed. I am not a big specialist in
> x86, but from what I said, I can see that there is no easy way to switch
> contexts while in hypervisor mode.
>
> Then I want to return to a task domain idea, which you mentioned in the
> other thread. If I got it right, it would allow to
>
> 1. Implement asynchronous hypercalls for cases when there is no reason
> to hold calling vCPU in hypervisor for the whole call duration
>

Okay, I was too overexcited there. I mean - surely it is possible to
implement async hypercalls, but there is no immediate profit in this:
such hypercall can't be preempted anyways. On a SMP system you can
offload hypercall to another core, but that's basically all.

> I skimmed through ML archives, but didn't found any discussion about it.

Maybe you can give some hint how to find it?

> As I see it, its implementation would be close to idle domain
> implementation, but a little different.


-- 
Volodymyr Babchuk at EPAM