On Wed, Feb 07, 2024, Xin Li wrote:
@@ -7382,6 +7419,24 @@ static noinstr void vmx_vcpu_enter_exit(struct
kvm_vcpu *vcpu,
vmx_disable_fb_clear(vmx);
- /*
* %cr2 needs to be saved after a VM exit and restored before a VM* entry in case a VM exit happens immediately after delivery of a* guest #PF but before guest reads %cr2.** A FRED guest should read its #PF faulting linear address from* the event data field in its FRED stack frame instead of %cr2.* But the FRED 5.0 spec still requires a FRED CPU to update %cr2* in the normal way, thus %cr2 is still updated even for a FRED* guest.** Note, an NMI could interrupt KVM:* 1) after VM exit but before CR2 is saved.* 2) after CR2 is restored but before VM entry.* And a #PF could happen durng NMI handlng, which overwrites %cr2.* Thus exc_nmi() should save and restore %cr2 upon entering and* before leaving to make sure %cr2 not corrupted.*/This is 99.9% noise. What software does or does not do with respect to CR2 is completely irrelevant. The *only* thing that matters is the architectural behavior, and architecturally guest CR2 _must_ be up-to-date at all times because CR2 accesses cannot be intercepted. So, just say:
/* * Note, even though FRED delivers the faulting linear address via the * event data field on the stack, CR2 is still updated. */
Will do!
There is a reason for this comment because it won't be architectural: https://lore.kernel.org/lkml/c0c7c605-d487-483e-a034-983b76ee1dfa@zytor.com/
FRED is designed to atomically save and restore _full_ supervisor/user context upon event delivery and return. But unfortunately, KVM still has to save/restore guest CR2 explicitly due to the issue mentioned above.
Thanks! Xin