linux-kernel - Re: [PATCH v2 2/3] KVM: x86/mmu: Fully re-evaluate MMIO caching when SPTE masks change

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Message-ID: <CALzav=e_H0LU+2-KcG_bPahVhJM8YGnH24J6aJ9HG9Eqj-waew@mail.gmail.com>
Date:   Fri, 19 Aug 2022 09:21:42 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kai Huang <kai.huang@...el.com>,
        Michael Roth <michael.roth@....com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 2/3] KVM: x86/mmu: Fully re-evaluate MMIO caching when
 SPTE masks change

On Wed, Aug 3, 2022 at 3:50 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Fully re-evaluate whether or not MMIO caching can be enabled when SPTE
> masks change; simply clearing enable_mmio_caching when a configuration
> isn't compatible with caching fails to handle the scenario where the
> masks are updated, e.g. by VMX for EPT or by SVM to account for the C-bit
> location, and toggle compatibility from false=>true.
>
> Snapshot the original module param so that re-evaluating MMIO caching
> preserves userspace's desire to allow caching.  Use a snapshot approach
> so that enable_mmio_caching still reflects KVM's actual behavior.

Is updating module parameters to reflect the actual behavior (vs.
userspace desire) something we should do for all module parameters?

I am doing an unrelated refactor to the tdp_mmu module parameter and
noticed it is not updated e.g. if userspace loads kvm_intel with
ept=N.