linux-kernel - Re: [PATCH RESEND v4 8/9] KVM: MMU: Enable Lazy mode SPPT setup

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Date:   Mon, 9 Sep 2019 19:10:22 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        sean.j.christopherson@...el.com, mst@...hat.com,
        rkrcmar@...hat.com, jmattson@...gle.com, yu.c.zhang@...el.com,
        alazar@...defender.com
Subject: Re: [PATCH RESEND v4 8/9] KVM: MMU: Enable Lazy mode SPPT setup

On 04/09/19 15:49, Yang Weijiang wrote:
>>> This would not enable SPP if the guest is backed by huge pages.
>>> Instead, either the PT_PAGE_TABLE_LEVEL level must be forced for all
>>> pages covered by SPP ranges, or (better) kvm_enable_spp_protection must
>>> be able to cover multiple pages at once.
>>>
>>> Paolo
>> OK, I'll figure out how to make it, thanks!
> Hi, Paolo,
> Regarding this change, I have some concerns, splitting EPT huge page
> entries(e.g., 1GB page)will take long time compared with normal EPT page
> fault processing, especially for multiple vcpus/pages,so the in-flight time increases,
> but HW walks EPT for translations in the meantime, would it bring any side effect? 
> or there's a way to mitigate it?

Sub-page permissions are only defined on EPT PTEs, not on large pages.
Therefore, in order to allow subpage permissions the EPT page tables
must already be split.

Paolo