linux-kernel - Re: [PATCH v7 00/11] KVM: xen: update shared_info and vcpu

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Date:   Thu, 09 Nov 2023 10:02:35 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     paul@....org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Paul Durrant <pdurrant@...zon.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org
Subject: Re: [PATCH v7 00/11] KVM: xen: update shared_info and vcpu_info
 handling

On Thu, 2023-10-05 at 09:36 +0100, Paul Durrant wrote:
> On 05/10/2023 07:41, David Woodhouse wrote:
> > On Mon, 2023-10-02 at 09:57 +0000, Paul Durrant wrote:
> > > From: Paul Durrant <pdurrant@...zon.com>
> > > 
> > > The following text from the original cover letter still serves as an
> > > introduction to the series:
> > > 
> > > "Currently we treat the shared_info page as guest memory and the VMM
> > > informs KVM of its location using a GFN. However it is not guest memory as
> > > such; it's an overlay page. So we pointlessly invalidate and re-cache a
> > > mapping to the *same page* of memory every time the guest requests that
> > > shared_info be mapped into its address space. Let's avoid doing that by
> > > modifying the pfncache code to allow activation using a fixed userspace HVA
> > > as well as a GPA."
> > > 
> > > This version of the series is functionally the same as version 6. I have
> > > simply added David Woodhouse's R-b to patch 11 to indicate that he has
> > > now fully reviewed the series.
> > 
> > Thanks. I believe Sean is probably waiting for us to stop going back
> > and forth, and for the dust to settle. So for the record: I think I'm
> > done heckling and this is ready to go in.
> > 
> > Are you doing the QEMU patches or am I?
> > 
> 
> I'll do the QEMU changes, once the patches hit kvm/next.

Note that I disabled migration support in QEMU for emulated Xen
guests. You might want that for testing, since the reason for this work
is to enable pause/serialize workflows.

Migration does work all the way up to XenStore itself, and
https://gitlab.com/qemu-project/qemu/-/commit/766804b101d *was* tested
with migration enabled. There are also unit tests for XenStore
serialize/deserialize.

I disabled it because the PV backends on the XenBus don't have
suspend/resume support. But a guest using other emulated net/disk
devices should still be able to suspend/resume OK if we just remove the
'unmigratable' flag from xen_xenstore, I believe. 

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