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

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Message-ID: <20190911002344.GA28130@local-michael-cet-test>
Date:   Wed, 11 Sep 2019 08:23:44 +0800
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, 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 Mon, Sep 09, 2019 at 07:10:22PM +0200, Paolo Bonzini wrote:
> 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
Thanks, I've added code to handle hugepage, will be included in
next version patch.