linux-kernel - Re: [PATCH 1/5] KVM: x86/xen: Restrict hypercall MSR to unofficial synthetic range

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Message-ID: <85f8aaea4cb5918cef92309c8c1c26fc7fd113b8.camel@infradead.org>
Date: Wed, 05 Feb 2025 17:15:42 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Paul Durrant <paul@....org>, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzbot+cdeaeec70992eca2d920@...kaller.appspotmail.com, Joao Martins
	 <joao.m.martins@...cle.com>
Subject: Re: [PATCH 1/5] KVM: x86/xen: Restrict hypercall MSR to unofficial
 synthetic range

On Wed, 2025-02-05 at 16:18 +0000, David Woodhouse wrote:
> 
> > Oh!  It doesn't help KVM avoid breaking userspace, but a way for QEMU to avoid a
> > future collision would be to have QEMU start at 0x40000200 when Hyper-V is enabled,
> > but then use KVM_GET_MSR_INDEX_LIST to detect a collision with KVM Hyper-V, e.g.
> > increment the index until an available index is found (with sanity checks and whatnot).
> 
> Makes sense. I think that's a third separate patch, yes?

To be clear, I think I mean a third patch which further restricts
kvm_xen_hvm_config() to disallow indices for which
kvm_is_advertised_msr() returns true?

We could roll that into your original patch instead, if you prefer.

Q: Should kvm_is_advertised_msr() include the Xen hypercall MSR, if one
is already configured? Life is easier if we answer 'no'...


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