Re: [RFC PATCH 1/1] xen: Use a global mapping for runstate

[Roger Pau Monné <roger@xxxxxxx>]

On Fri, May 29, 2020 at 09:18:42AM +0000, Bertrand Marquis wrote:
> Hi Jan,
> 
> > On 29 May 2020, at 09:45, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> > 
> > On 29.05.2020 10:13, Bertrand Marquis wrote:
> >>> On 28 May 2020, at 19:54, Julien Grall <julien@xxxxxxx> wrote:
> >>> AFAICT, there is no restriction on when the runstate hypercall can be 
> >>> called. So this can even be called before the vCPU is brought up.
> >> 
> >> I understand the remark but it still feels very weird to allow an invalid 
> >> address in an hypercall.
> >> Wouldn’t we have a lot of potential issues accepting an address that we 
> >> cannot check ?
> > 
> > I don't think so: The hypervisor uses copy_to_guest() to protect
> > itself from the addresses to be invalid at the time of copying.
> > If the guest doesn't make sure they're valid at that time, it
> > simply won't get the information (perhaps until Xen's next
> > attempt to copy it out).
> > 
> > You may want to take a look at the x86 side of this (also the
> > vCPU time updating): Due to the way x86-64 PV guests work, the
> > address may legitimately be unmapped at the time Xen wants to
> > copy it, when the vCPU is currently executing guest user mode
> > code. In such a case the copy gets retried the next time the
> > guest transitions from user to kernel mode (which involves a
> > page table change).
> 
> If I understand everything correctly runstate is updated only if there is
> a context switch in xen while the guest is running in kernel mode and
> if the address is mapped at that time.
> 
> So this is a best effort in Xen and the guest cannot really rely on the
> runstate information (as it might not be up to date).
> Could this have impacts somehow if this is used for scheduling ?
> 
> In the end the only accepted trade off would be to:
> - reduce error verbosity and just ignore it
> - introduce a new system call using a physical address
>   -> Using a virtual address with restrictions sounds very complex
>   to document (current core, no remapping).
> 
> But it feels like having only one hypercall using guest physical addresses
> would not really be logic and this kind of change should be made across
> all hypercalls if it is done.

FRT, there are other hypercalls using a physical address instead of a
linear one, see VCPUOP_register_vcpu_info for example. It's just a
mixed bag right now, with some hypercalls using a linear address and
some using a physical one.

I think introducing a new hypercall that uses a physical address would
be fine, and then you can add a set of restrictions similar to the
ones listed by VCPUOP_register_vcpu_info.

Changing the current hypercall as proposed is risky, but I think the
current behavior is broken by design specially on auto translated
guests, even more with XPTI.

Thanks, Roger.