Re: [PATCH v13 24/35] x86/fred: Add a NMI entry stub for FRED

["H. Peter Anvin" <hpa@xxxxxxxxx>]

So we have recently discovered an overlooked interaction with VT-x. 
Immediately before VMENTER and after VMEXIT, CR2 is live with the 
*guest* CR2. Regardless of if the guest uses FRED or not, this is guest 
state and SHOULD NOT be corrupted. Furthermore, host state MUST NOT leak 
into the guest.
NMIs are blocked on VMEXIT if the cause was an NMI, but not for other 
reasons, so a NMI coming in during this window that then #PFs could 
corrupt the guest CR2.
Intel is exploring ways to close this hole, but for schedule reasons, it 
will not be available at the same time as the first implementation of 
FRED. Therefore, as a workaround, it turns out that the FRED NMI stub 
*will*, unfortunately, have to save and restore CR2 after all when (at 
least) Intel KVM is in use.
Note that this is airtight: it does add a performance penalty to the NMI 
path (two read CR2 in the common case of no #PF), but there is no gap 
during which a bad CR2 value could be introduced in the guest, no matter 
in which sequence the events happen.
In theory the performance penalty could be further reduced by 
conditionalizing this on the NMI happening in the critical region in the 
KVM code, but it seems to be pretty far from necessary to me.
This obviously was an unfortunate oversight on our part, but the 
workaround is simple and doesn't affect any non-NMI paths.
        -hpa

On 12/5/23 02:50, Xin Li wrote:
> +
> +       if (IS_ENABLED(CONFIG_SMP) && arch_cpu_is_offline(smp_processor_id()))
> +               return;
> +
This is cut & paste from elsewhere in the NMI code, but I believe the 
IS_ENABLED() is unnecessary (not to mention ugly): smp_processor_id() 
should always return zero on UP, and arch_cpu_is_offline() reduces to 
!(cpu == 0), so this is a statically true condition on UP.
        -hpa