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 20:49:43 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.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 Wed, 2021-02-17 at 18:37 +0100, 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 agree with that.

> 
> 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.

I was thinking about this a well, I tried to make an as simple as possible
solution that doesn't make things worse.
> 
> 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].

I agree with all of this. I'll see why this code is needed (it is needed,
since I once removed it accidentaly on VMX, and it broke nesting with ept=0,
in exact the same way as it was broken on AMD).

I''l debug this a bit to see if I can make it work as you suggest.


Best regards,
	Maxim Levitsky
> 
> 
> Paolo
>