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: <bab3cc28-4473-d446-bb6d-bca6939adb63@redhat.com>
Date:   Fri, 19 Aug 2022 19:37:33 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Matlack <dmatlack@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     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 8/19/22 18:21, David Matlack wrote:
> 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.

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