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

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Message-ID: <2d43d29f-d29c-3dd7-1709-4414f34d27da@redhat.com>
Date:   Thu, 1 Oct 2020 01:34:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Eric van Tassell <evantass@....com>,
        Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Cannon Matthews <cannonmatthews@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.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 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.

Paolo