Re: [Xen-devel] How to deal with hypercalls returning -EFAULT

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.06.18 at 18:25, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 13/06/18 16:27, Juergen Gross wrote:
>> 4. Modify the interface between hypervisor and kernel: instead of just
>>    returning -EFAULT let the hypervisor behave more like copy_to_user by
>>    raising a page fault which can then be fixed up in the kernel. This
>>    change must be activated by the kernel, of course.
>>    Pros: - rather simple change in the kernel "doing the right thing"
>>          - hypercall bounce buffer handling in libxc/libxencall can be
>>            switched off for a kernel supporting this chnage
>>    Cons: - split solution between kernel and Xen, both must be changed
>>          - not sure how complex the required hypervisor change will be
> 
> Sadly, as I've just realised...
> 
> Con: Cannot be used to replace all -EFAULTs.
> 
> Faults when copying data in can be resolved by passing #PF to the
> kernel, but faults when trying to update guest state (continuation, or
> completion information) cannot be safely resumed at a later point.

Yeah, I've too realized this meanwhile. This is not an option, at least
not in simple enough a shape as would be necessary here.

Jan



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