linux-kernel - Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND

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Date:   Wed, 8 Apr 2020 08:34:14 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Vivek Goyal <vgoyal@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>, stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

On Wed, Apr 08, 2020 at 10:23:58AM +0200, Paolo Bonzini wrote:
> Page-not-present async page faults are almost a perfect match for the
> hardware use of #VE (and it might even be possible to let the processor
> deliver the exceptions).

My "async" page fault knowledge is limited, but if the desired behavior is
to reflect a fault into the guest for select EPT Violations, then yes,
enabling EPT Violation #VEs in hardware is doable.  The big gotcha is that
KVM needs to set the suppress #VE bit for all EPTEs when allocating a new
MMU page, otherwise not-present faults on zero-initialized EPTEs will get
reflected.

Attached a patch that does the prep work in the MMU.  The VMX usage would be:

	kvm_mmu_set_spte_init_value(VMX_EPT_SUPPRESS_VE_BIT);

when EPT Violation #VEs are enabled.  It's 64-bit only as it uses stosq to
initialize EPTEs.  32-bit could also be supported by doing memcpy() from
a static page.

View attachment "0001-KVM-x86-mmu-Allow-non-zero-init-value-for-shadow-PTE.patch" of type "text/x-diff" (5342 bytes)