linux-kernel - Re: [PATCH 02/22] kvm: mmu: Introduce tdp

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Message-ID: <20201001000752.GB2988@linux.intel.com>
Date:   Wed, 30 Sep 2020 17:07:52 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Eric van Tassell <evantass@....com>,
        Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Cannon Matthews <cannonmatthews@...gle.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 02/22] kvm: mmu: Introduce tdp_iter

On Thu, Oct 01, 2020 at 01:34:53AM +0200, Paolo Bonzini wrote:
> On 01/10/20 01:20, Eric van Tassell wrote:
> >>
> >> +int is_shadow_present_pte(u64 pte)
> >>   {
> >>       return (pte != 0) && !is_mmio_spte(pte);
> > From <Figure 28-1: Formats of EPTP and EPT Paging-Structure Entries" of
> > the manual I don't have at my fingertips right now, I believe you should
> > only check the low 3 bits(mask = 0x7). Since the upper bits are ignored,
> > might that not mean they're not guaranteed to be 0?
> 
> No, this a property of the KVM MMU (and how it builds its PTEs) rather
> than the hardware present check.

Ya, I found out the hard way that "present" in is_shadow_present_pte() really
means "valid", or "may be present".  The most notable case is EPT without A/D
bits (I think this is the only case where a valid SPTE can be fully not-present
in hardware).  Accessed tracking will clear all RWX bits to make the EPT entry
not-present, but from KVM's perspective it's treated as valid/present because
it can be made present in hardware without taking the MMU lock.