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Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
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Date:   Fri, 19 Aug 2022 19:37:33 +0200
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Subject: Re: [PATCH v2 2/3] KVM: x86/mmu: Fully re-evaluate MMIO caching when
 SPTE masks change
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To:     David Matlack <dmatlack@google.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kai Huang <kai.huang@intel.com>,
        Michael Roth <michael.roth@amd.com>,
        Tom Lendacky <thomas.lendacky@amd.com>
References: <20220803224957.1285926-1-seanjc@google.com>
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 <CALzav=e_H0LU+2-KcG_bPahVhJM8YGnH24J6aJ9HG9Eqj-waew@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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On 8/19/22 18:21, David Matlack wrote:
> On Wed, Aug 3, 2022 at 3:50 PM Sean Christopherson <seanjc@google.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.

If it is cheap/easy then yeah, updating the parameters is the right 
thing to do.  Generally, however, this is only done for 
kvm_intel/kvm_amd modules that depend on hardware features, because they 
are more important for debugging user issues.  (Or at least they were 
until vmx features were added to /proc/cpuinfo).

Paolo