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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Sean Christopherson <seanjc@google.com>,
        Andrei Vagin <avagin@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, Wanpeng Li <wanpengli@tencent.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Jianfeng Tan <henry.tjf@antfin.com>,
        Adin Scannell <ascannell@google.com>,
        Konstantin Bogomolov <bogomolov@google.com>,
        Etienne Perot <eperot@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 0/5] KVM/x86: add a new hypercall to execute host system
In-Reply-To: <YuAD6qY+F2nuGm62@google.com>
References: <20220722230241.1944655-1-avagin@google.com>
 <Yts1tUfPxdPH5XGs@google.com>
 <CAEWA0a4hrRb5HYLqa1Q47=guY6TLsWSJ_zxNjOXXV2jCjUekUA@mail.gmail.com>
 <YuAD6qY+F2nuGm62@google.com>
Date:   Wed, 27 Jul 2022 00:10:12 +0200
Message-ID: <875yjjttiz.ffs@tglx>
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On Tue, Jul 26 2022 at 15:10, Sean Christopherson wrote:
> On Tue, Jul 26, 2022, Andrei Vagin wrote:
>> * It doesn't allow to support Confidential Computing (SEV-ES/SGX). The Sentry
>>   has to be fully enclosed in a VM to be able to support these technologies.
>
> Speaking of SGX, this reminds me a lot of Graphene, SCONEs, etc..., which IIRC
> tackled the "syscalls are crazy expensive" problem by using a message queue and
> a dedicated task outside of the enclave to handle syscalls.  Would something like
> that work, or is having to burn a pCPU (or more) to handle syscalls in the host a
> non-starter?

Let's put VMs aside for a moment. The problem you are trying to solve is
ptrace overhead because that requires context switching, right?

Did you ever try to solve this with SYSCALL_USER_DISPATCH? That requires
signals, which are not cheap either, but we certainly could come up with
a lightweight signal implementation for that particular use case.

Thanks,

        tglx