Thomas Gleixner tglx@linutronix.de writes:
Thomas Gleixner tglx@linutronix.de writes:
Andy Lutomirski luto@kernel.org writes:
On Sat, Mar 7, 2020 at 7:47 AM Thomas Gleixner tglx@linutronix.de wrote:
The host knows exactly when it injects a async PF and it can store CR2 and reason of that async PF in flight.
On the next VMEXIT it checks whether apf_reason is 0. If apf_reason is 0 then it knows that the guest has read CR2 and apf_reason. All good nothing to worry about.
If not it needs to be careful.
As long as the apf_reason of the last async #PF is not cleared by the guest no new async #PF can be injected. That's already correct because in that case IF==0 which prevents a nested async #PF.
If MCE, NMI trigger a real pagefault then the #PF injection needs to clear apf_reason and set the correct CR2. When that #PF returns then the old CR2 and apf_reason need to be restored.
How is the host supposed to know when the #PF returns? Intercepting IRET sounds like a bad idea and, in any case, is not actually a reliable indication that #PF returned.
The host does not care about the IRET. It solely has to check whether apf_reason is 0 or not. That way it knows that the guest has read CR2 and apf_reason.
Bah. I'm a moron. Of course it needs to trap the IRET of the #PF in order to restore CR2 and apf_reason. Alternatively it could trap the CR2 read of #PF, but yes that's all nasty.
Some hours or sleep and not staring at this meess later and while reading the leaves of my morning tea:
guest side:
nmi()/mce() ...
stash_crs();
+ stash_and_clear_apf_reason();
....
+ restore_apf_reason();
restore_cr2();
Too obvious, isn't it?
Thanks,
tglx