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Subject: Re: [PATCH v2 07/20] kvm: x86/mmu: Support zapping SPTEs in the TDP
 MMU
To:     Yu Zhang <yu.c.zhang@linux.intel.com>
Cc:     Ben Gardon <bgardon@google.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, Cannon Matthews <cannonmatthews@google.com>,
        Peter Xu <peterx@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Peter Shier <pshier@google.com>,
        Peter Feiner <pfeiner@google.com>,
        Junaid Shahid <junaids@google.com>,
        Jim Mattson <jmattson@google.com>,
        Yulei Zhang <yulei.kernel@gmail.com>,
        Wanpeng Li <kernellwp@gmail.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Xiao Guangrong <xiaoguangrong.eric@gmail.com>
References: <20201014182700.2888246-1-bgardon@google.com>
 <20201014182700.2888246-8-bgardon@google.com>
 <20201021150225.2eeriqlffqnsm4b3@linux.intel.com>
 <6985f630-3b2a-75f5-5b55-bd76cf32f20b@redhat.com>
 <20201021172409.aids3y2mlyx776lx@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <0f2e00fe-4f30-88d3-e345-089f1afc4fb9@redhat.com>
Date:   Wed, 21 Oct 2020 20:00:47 +0200
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On 21/10/20 19:24, Yu Zhang wrote:
> On Wed, Oct 21, 2020 at 07:20:15PM +0200, Paolo Bonzini wrote:
>> On 21/10/20 17:02, Yu Zhang wrote:
>>>>  void kvm_tdp_mmu_free_root(struct kvm *kvm, struct kvm_mmu_page *root)
>>>>  {
>>>> +	gfn_t max_gfn = 1ULL << (boot_cpu_data.x86_phys_bits - PAGE_SHIFT);
>>>> +
>>> boot_cpu_data.x86_phys_bits is the host address width. Value of the guest's
>>> may vary. So maybe we should just traverse the memslots and zap the gfn ranges
>>> in each of them?
>>>
>>
>> It must be smaller than the host value for two-dimensional paging, though.
> 
> Yes. And using boot_cpu_data.x86_phys_bits works, but won't it be somewhat
> overkilling? E.g. for a host with 46 bits and a guest with 39 bits width?

It would go quickly through extra memory space because the PML4E entries
above the first would be empty.  So it's just 511 comparisons.

Paolo