Re: [PATCH V2] KVM: X86: Move PTE present check from loop body to __shadow_walk_next()

From: Sean Christopherson
Date: Thu Sep 02 2021 - 16:43:33 EST

On Fri, Aug 13, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> So far, the loop bodies already ensure the PTE is present before calling
> __shadow_walk_next(): Some loop bodies simply exit with a !PRESENT
> directly and some other loop bodies, i.e. FNAME(fetch) and __direct_map()
> do not currently terminate their walks with a !PRESENT, but they get away
> with it because they install present non-leaf SPTEs in the loop itself.
> But checking pte present in __shadow_walk_next() is a more prudent way of
> programing and loop bodies will not need to always check it. It allows us
> removing unneded is_shadow_present_pte() in the loop bodies.

> Terminating on !is_shadow_present_pte() is 100% the correct behavior, as
> walking past a !PRESENT SPTE would lead to attempting to read a the next
> level SPTE from a garbage iter->shadow_addr. Even some paths that do _not_
> currently have a !is_shadow_present_pte() in the loop body is Ok since
> they will install present non-leaf SPTEs and the additinal present check
> is just an NOP.
> The checking result in __shadow_walk_next() will be propagated to
> shadow_walk_okay() for being used in any for(;;) loop.
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> ---

Nits aside,

Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>