On 09/03/20 07:57, Thomas Gleixner wrote:
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:
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.
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.
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?
Yes, this works but Andy was not happy about adding more save-and-restore to NMIs. If you do not want to do that, I'm okay with disabling async page fault support for now.
Storing the page fault reason in memory was not a good idea. Better options would be to co-opt the page fault error code (e.g. store the reason in bits 31:16, mark bits 15:0 with the invalid error code RSVD=1/P=0), or to use the virtualization exception area.
Paolo