Re: [Xen-devel] [PATCH V2 2/2] x86/vm_event: fix race between __context_switch() and vm_event_resume()

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 05/03/17 13:01, Jan Beulich wrote:
>>>> On 03.05.17 at 11:10, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> The introspection agent can reply to a vm_event faster than
>> vmx_vmexit_handler() can complete in some cases, where it is then
>> not safe for vm_event_set_registers() to modify v->arch.user_regs.
>> In the test scenario, we were stepping over an INT3 breakpoint by
>> setting RIP += 1. The quick reply tended to complete before the VCPU
>> triggering the introspection event had properly paused and been
>> descheduled. If the reply occurs before __context_switch() happens,
>> __context_switch() clobbers the reply by overwriting
>> v->arch.user_regs from the stack. If the reply occurs after
>> __context_switch(), we don't pass through __context_switch() when
>> transitioning to idle.
> 
> This last sentence still looks to be wrong (and even self-contradictory).
> The reply can't occur after __context_switch() if we don't make it there
> in the first place. How about "If we don't pass through
> __context_switch() (due to switching to the idle vCPU), reply data
> wouldn't be picked up when switching back straight to the original
> vCPU"?

Quite right, it's very convoluted. I'll update the comment.


Thanks,
Razvan


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