linux-kernel - Re: [PATCH 03/16] KVM: x86: set gfn-to-pfn cache length consistently with VM word size

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Date:   Mon, 14 Nov 2022 17:58:01 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        "seanjc@...gle.com" <seanjc@...gle.com>
Cc:     "mhal@...x.co" <mhal@...x.co>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Durrant, Paul" <pdurrant@...zon.co.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Kaya, Metin" <metikaya@...zon.co.uk>
Subject: Re: [PATCH 03/16] KVM: x86: set gfn-to-pfn cache length consistently
 with VM word size

On 11/14/22 15:53, Woodhouse, David wrote:
> Most other data structures, including the pvclock info (both Xen and
> native KVM), could potentially cross page boundaries. And isn't that
> also true for things that we'd want to use the GPC for in nesting?

Yes, for kvmclock we likely got away with it because Linux page-aligns 
it (and has been since 2013: commit ed55705dd, originally done for 
vsyscall support).  I have checked OpenBSD and FreeBSD and I think they 
do as well.

I am very very tempted to remove support for "old-style" kvmclock MSRs 
and retroactively declare new-style MSRs to accept only 32-byte aligned 
addresses.  However that doesn't solve the problem.

Paolo