On Tue, 2023-06-06 at 17:18 +0800, Binbin Wu wrote:Here is the discussion about the general rule of interception of CR4 bit.
Move CR4.LAM_SUP out of CR4_RESERVED_BITS and its reservation depends on vcpuKVM only needs to do vmread once to cache guest's CR4, and presumable vmread is
supporting LAM feature or not. Leave the bit intercepted to avoid vmread every
time when KVM fetches its value, with the expectation that guest won't toggle
the bit frequently.
a lot cheaper than a VMEXIT. So I don't see the value of intercepting it if
there's no need to do.
Right, this is also a reason why the CR4.LAM_SUP bit should be intercepted.
But presumably I think we cannot allow guest to own this bit because KVM wants
to return a valid CR4 if LAM isn't exposed to guest? Otherwise guest can still
set this bit even LAM isn't exposed to guest.
Am I missing something?
If not, your justification of intercepting this bit isn't correct and needs
update.