linux-kernel - Re: [PATCH V2] KVM: X86: Move PTE present check from loop body to __shadow_walk

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Date:   Thu, 2 Sep 2021 20:43:25 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org
Subject: Re: [PATCH V2] KVM: X86: Move PTE present check from loop body to
 __shadow_walk_next()

On Fri, Aug 13, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> 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.
           ^^^^^^^
	   unneeded

> 
> 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
                                                   ^^^^^^^^^
						   additional
> 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@...ux.alibaba.com>
> ---

Nits aside,

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