Re: [PATCH] kvm: x86: mmu: Drop the need_remote_flush() function

[Sean Christopherson <seanjc@google.com>]

On Mon, Jul 25, 2022, David Matlack wrote:
> On Fri, Jul 22, 2022 at 07:43:16PM -0700, Junaid Shahid wrote:
> > This is only used by kvm_mmu_pte_write(), which no longer actually
> > creates the new SPTE and instead just clears the old SPTE. So we
> > just need to check if the old SPTE was shadow-present instead of
> > calling need_remote_flush(). Hence we can drop this function. It was
> > incomplete anyway as it didn't take access-tracking into account.
> > 
> > This patch should not result in any functional change.
> 
> Even if we don't assume anything about the new SPTE, this commit flushes
> TLBs in a superset of the current cases. So this change should
> definitely be safe.
> 
> And then if we assume new SPTE is 0 (which it should be), I agree this
> should not result in any functional change.

Nit for posterity, zapped SPTEs don't necessarily have to be '0', e.g. KVM is
more than likely going to use 0x80000000_00000000 as the "zero" value for TDP MMU
SPTEs so that the SUPPRESS_VE is set for "zero"-initialized SPTEs (TDX requires
EPT Violation #VEs be enabled).

> Reviewed-by: David Matlack <dmatlack@google.com>

Reviewed-by: Sean Christopherson <seanjc@google.com>