linux-kernel - Re: [PATCH 4/7] KVM: nVMX: move inject_page_fault tweak to .complete_mmu

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Date:   Wed, 17 Feb 2021 09:57:55 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>,
        Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 4/7] KVM: nVMX: move inject_page_fault tweak to
 .complete_mmu_init

On Wed, Feb 17, 2021, Paolo Bonzini wrote:
> On 17/02/21 18:29, Sean Christopherson wrote:
> > All that being said, I'm pretty we can eliminate setting
> > inject_page_fault dynamically. I think that would yield more
> > maintainable code. Following these flows is a nightmare. The change
> > itself will be scarier, but I'm pretty sure the end result will be a lot
> > cleaner.
> 
> I had a similar reaction, though my proposal was different.
> 
> The only thing we're changing in complete_mmu_init is the page fault
> callback for init_kvm_softmmu, so couldn't that be the callback directly
> (i.e. something like context->inject_page_fault =
> kvm_x86_ops.inject_softmmu_page_fault)?  And then adding is_guest_mode to
> the conditional that is already in vmx_inject_page_fault_nested and
> svm_inject_page_fault_nested.

Heh, that exact code crossed my mind as well.

> That said, I'm also rusty on _why_ this code is needed.  Why isn't it enough
> to inject the exception normally, and let nested_vmx_check_exception decide
> whether to inject a vmexit to L1 or an exception into L2?

Hmm, I suspect it was required at one point due to deficiencies elsewhere.
Handling this in the common fault handler logic does seem like the right
approach.

> Also, bonus question which should have been in the 5/7 changelog: are there
> kvm-unit-tests testcases that fail with npt=0, and if not could we write
> one?  [Answer: the mode_switch testcase fails, but I haven't checked why].
> 
> 
> Paolo
>