linux-kernel - Re: [PATCH] KVM: x86: Add kvm_x86_ops hook to short circuit emulation

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Date:   Tue, 22 Sep 2020 15:32:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH] KVM: x86: Add kvm_x86_ops hook to short circuit emulation

On 16/09/20 01:27, Sean Christopherson wrote:
> Replace the existing kvm_x86_ops.need_emulation_on_page_fault() with a
> more generic is_emulatable(), and unconditionally call the new function
> in x86_emulate_instruction().
> 
> KVM will use the generic hook to support multiple security related
> technologies that prevent emulation in one way or another.  Similar to
> the existing AMD #NPF case where emulation of the current instruction is
> not possible due to lack of information, AMD's SEV-ES and Intel's SGX
> and TDX will introduce scenarios where emulation is impossible due to
> the guest's register state being inaccessible.  And again similar to the
> existing #NPF case, emulation can be initiated by kvm_mmu_page_fault(),
> i.e. outside of the control of vendor-specific code.
> 
> While the cause and architecturally visible behavior of the various
> cases are different, e.g. SGX will inject a #UD, AMD #NPF is a clean
> resume or complete shutdown, and SEV-ES and TDX "return" an error, the
> impact on the common emulation code is identical: KVM must stop
> emulation immediately and resume the guest.
> 
> Query is_emulatable() in handle_ud() as well so that the
> force_emulation_prefix code doesn't incorrectly modify RIP before
> calling emulate_instruction() in the absurdly unlikely scenario that
> KVM encounters forced emulation in conjunction with "do not emulate".

Ahah, I love those adverb + adjective pairs (my favorite is
https://patchwork.kernel.org/cover/10710751/ which unfortunately was in
the cover letter and thus was not immortalized in linux.git).

"is_emulatable" is not very grammatical, so I'd rather call it
"can_emulate_instruction" instead.  Either way, it's an improvement over
"need".  Queued, thanks.

Paolo