From: Zhong, Yang <redacted>
Sent: Wednesday, December 29, 2021 9:13 PM

Highly appreciate for your review. This version mostly addressed the
comments
from Sean. Most comments are adopted except three which are not closed
and
need more discussions:

  - Move the entire xfd write emulation code to x86.c. Doing so requires
    introducing a new kvm_x86_ops callback to disable msr write bitmap.
    According to Paolo's earlier comment he prefers to handle it in vmx.c.

  - Directly check msr_bitmap in update_exception_bitmap() (for
    trapping #NM) and vcpu_enter_guest() (for syncing guest xfd after
    vm-exit) instead of introducing an extra flag in the last patch. However,
    doing so requires another new kvm_x86_ops callback for checking
    msr_bitmap since vcpu_enter_guest() is x86 common code. Having an
    extra flag sounds simpler here (at least for the initial AMX support).
    It does penalize nested guest with one xfd sync per exit, but it's not
    worse than a normal guest which initializes xfd but doesn't run
    AMX applications at all. Those could be improved afterwards.

Another option is to move xfd sync into vmx_vcpu_run(), given that
disabling xfd interception is vmx specific code thus it makes some
sense to also handle related side-effect in vmx.c. If this can be agreed
then yes there is no need of an extra flag and just checking msr_bitmap
is sufficient (and more accurate).

Paolo, how is your opinion?

  - Disable #NM trap for nested guest. This version still chooses to always
    trap #NM (regardless in L1 or L2) as long as xfd write interception is
disabled.
    In reality #NM is rare if nested guest doesn't intend to run AMX
applications
    and always-trap is safer than dynamic trap for the basic support in case
    of any oversight here.

Sean just pointed out some potential issues in current logic, about handling
#NM raised in L2 guest (could happen just due to L1 interception).

It's being discussed here:

	https://lore.kernel.org/all/YcyaN7V4wwGI7wDV@google.com/ (local)

Thanks
Kevin