linux-kernel - Re: [PATCH v4] x86/hyperv: Fix kdump on Azure CVMs

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Message-ID: <aNxeO8wfvY6MvOos@liuwe-devbox-ubuntu-v2.lamzopl0uupeniq2etz1fddiyg.xx.internal.cloudapp.net>
Date: Tue, 30 Sep 2025 22:48:27 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Michael Kelley <mhklinux@...look.com>
Cc: Vitaly Kuznetsov <vkuznets@...hat.com>, Wei Liu <wei.liu@...nel.org>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Dexuan Cui <decui@...rosoft.com>, "x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
	Tianyu Lan <tiala@...rosoft.com>, Li Tian <litian@...hat.com>,
	Philipp Rudo <prudo@...hat.com>
Subject: Re: [PATCH v4] x86/hyperv: Fix kdump on Azure CVMs

On Fri, Sep 05, 2025 at 03:57:03PM +0000, Michael Kelley wrote:
> From: Vitaly Kuznetsov <vkuznets@...hat.com> Sent: Friday, September 5, 2025 12:48 AM
> > 
> > Wei Liu <wei.liu@...nel.org> writes:
> > 
> > > On Thu, Aug 28, 2025 at 12:16:18PM +0300, Vitaly Kuznetsov wrote:
> > >> Azure CVM instance types featuring a paravisor hang upon kdump. The
> > >> investigation shows that makedumpfile causes a hang when it steps on a page
> > >> which was previously share with the host
> > >> (HVCALL_MODIFY_SPARSE_GPA_PAGE_HOST_VISIBILITY). The new kernel has no
> > >> knowledge of these 'special' regions (which are Vmbus connection pages,
> > >> GPADL buffers, ...). There are several ways to approach the issue:
> > >> - Convey the knowledge about these regions to the new kernel somehow.
> > >> - Unshare these regions before accessing in the new kernel (it is unclear
> > >> if there's a way to query the status for a given GPA range).
> > >> - Unshare these regions before jumping to the new kernel (which this patch
> > >> implements).
> > >>
> > >> To make the procedure as robust as possible, store PFN ranges of shared
> > >> regions in a linked list instead of storing GVAs and re-using
> > >> hv_vtom_set_host_visibility(). This also allows to avoid memory allocation
> > >> on the kdump/kexec path.
> > >>
> > >> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> > >
> > > No fixes tag for this one?
> > >
> > 
> > Personally, I don't see this as a 'bug', it's rather a missing
> > feature. In theory, we can add something like
> > 
> > Fixes: 810a52126502 ("x86/hyperv: Add new hvcall guest address host visibility
> > support")
> > 
> > but I'm on the fence whether this is accurate or not.
> > 
> > > Should it be marked as a stable backport?
> > 
> > I think it may make sense even without an explicit 'Fixes:': kdump is the
> > user's last resort when it comes to kernel crashes and doubly so on
> > CVMs. Pure kexec may also come handy.
> > 
> 
> I agree -- think of this as adding a feature instead of fixing a bug. Prior
> to now, there hasn't been any attempt to make kexec/kdump work
> for Azure CVMs.
> 
> Instead of using the word "Fix", maybe the patch "Subject:" should be
> changed to "x86/hyperv: Add kexec/kdump support on Azure CVMs".

I have fixed that in the hyperv-next tree. Thanks for the discussion.

Wei

> 
> Michael