linux-kernel - Re: [PATCH 0/5] KVM/x86: add a new hypercall to execute host system

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Message-ID: <875yjjttiz.ffs@tglx>
Date:   Wed, 27 Jul 2022 00:10:12 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sean Christopherson <seanjc@...gle.com>,
        Andrei Vagin <avagin@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jianfeng Tan <henry.tjf@...fin.com>,
        Adin Scannell <ascannell@...gle.com>,
        Konstantin Bogomolov <bogomolov@...gle.com>,
        Etienne Perot <eperot@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 0/5] KVM/x86: add a new hypercall to execute host system

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