linux-kernel - Re: [PATCH v2 07/20] kvm: x86/mmu: Support zapping SPTEs in the TDP MMU

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Date:   Wed, 21 Oct 2020 20:00:47 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>
Cc:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, 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 v2 07/20] kvm: x86/mmu: Support zapping SPTEs in the TDP
 MMU

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