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

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Message-ID: <ce28e893-2ed0-ea6f-6c36-b08bb0d814f2@redhat.com>
Date:   Wed, 8 Apr 2020 18:50:22 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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 08/04/20 17:34, Sean Christopherson wrote:
> 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.

The complication is that (at least according to the current ABI) we
would not want #VE to kick if the guest currently has IF=0 (and possibly
CPL=0).  But the ABI is not set in stone, and anyway the #VE protocol is
a decent one and worth using as a base for whatever PV protocol we design.

Paolo