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 18:37:25 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     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 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.

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?

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