linux-kernel - Re: [RFC 05/33] KVM: x86: hyper-v: Introduce VTL call/return prologues in hypercall page

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Message-ID: <ZWoKlJUKJGGhRRgM@google.com>
Date:   Fri, 1 Dec 2023 08:32:20 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Nicolas Saenz Julienne <nsaenz@...zon.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, anelkz@...zon.com,
        graf@...zon.com, dwmw@...zon.co.uk, jgowans@...zon.com,
        kys@...rosoft.com, haiyangz@...rosoft.com, decui@...rosoft.com,
        x86@...nel.org, linux-doc@...r.kernel.org
Subject: Re: [RFC 05/33] KVM: x86: hyper-v: Introduce VTL call/return
 prologues in hypercall page

On Fri, Dec 01, 2023, Nicolas Saenz Julienne wrote:
> > To support this I think that we can add a userspace msr filter on the HV_X64_MSR_HYPERCALL,
> > although I am not 100% sure if a userspace msr filter overrides the in-kernel msr handling.
> 
> I thought about it at the time. It's not that simple though, we should
> still let KVM set the hypercall bytecode, and other quirks like the Xen
> one.

Yeah, that Xen quirk is quite the killer.

Can you provide pseudo-assembly for what the final page is supposed to look like?
I'm struggling mightily to understand what this is actually trying to do.