Re: [PATCH 4/4] KVM: X86: Only get rflags when needed in permission_fault()

[Sean Christopherson]

On Tue, Dec 07, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> 
> In same cases, it doesn't need to get rflags for SMAP checks.
> 
> For example: it is user mode access, it could have contained other
> permission fault, SMAP is not enabled, it is implicit supervisor
> access, or it is nested TDP pagetable.

I don't disagree that reading RFLAGS is silly and _may_ have worse performance,
but I'd prefer any change have actual numbers to justify that it's an improvement
or at least a wash / in the noise.

Too much of the MMU code (and KVM in general) has optimizations like this that
have bitrotted horribly over the years.  And in many/most cases, the original commit
didn't provide performance numbers, so it's not even clear that the "optimizations"
were _ever_ a net win.